Implement ChannelList for new chat design

Reference design: https://www.figma.com/file/f8b2dHp9LJCMOqYP4mdrPZ/Client%2FChat?node-id=1%3A297

Adds new component `ChannelList` which makes up the sidebar channel
selector of the new chat design. Contains two separate fill flows for
public and private `ChannelItem` child components.

Exposed bindable `SelectorActive` to indicate current state of the "Add
more channels" button. Requires `Bindable<Channel>` from parent
component.

Renames and updates the `TestSceneChannelListItem` to
`TestSceneChannelList` to make use of new component and having both
tests seemed redundant.

osu.Game/Overlays/Chat/ChannelList/ChannelList.cs

@@ -0,0 +1,174 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using osu.Framework.Allocation;
+using osu.Framework.Bindables;
+using osu.Framework.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Shapes;
+using osu.Framework.Input.Events;
+using osu.Game.Graphics;
+using osu.Game.Graphics.Containers;
+using osu.Game.Graphics.Sprites;
+using osu.Game.Online.Chat;
+using osuTK;
+
+namespace osu.Game.Overlays.Chat.ChannelList
+{
+    public class ChannelList : Container
+    {
+        public Action<Channel>? OnRequestSelect;
+        public Action<Channel>? OnRequestLeave;
+
+        public readonly BindableBool SelectorActive = new BindableBool();
+
+        private readonly Dictionary<Channel, ChannelListItem> channelMap = new Dictionary<Channel, ChannelListItem>();
+
+        private ChannelListItemFlow publicChannelFlow = null!;
+        private ChannelListItemFlow privateChannelFlow = null!;
+
+        [BackgroundDependencyLoader]
+        private void load(OverlayColourProvider colourProvider)
+        {
+            Children = new Drawable[]
+            {
+                new Box
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Colour = colourProvider.Background6,
+                },
+                new ChannelListScrollContainer
+                {
+                    Padding = new MarginPadding { Vertical = 7 },
+                    RelativeSizeAxes = Axes.Both,
+                    ScrollbarAnchor = Anchor.TopLeft,
+                    ScrollDistance = 35f,
+                    Child = new FillFlowContainer
+                    {
+                        Direction = FillDirection.Vertical,
+                        RelativeSizeAxes = Axes.X,
+                        AutoSizeAxes = Axes.Y,
+                        Children = new Drawable[]
+                        {
+                            publicChannelFlow = new ChannelListItemFlow("CHANNELS"),
+                            new ChannelListSelector
+                            {
+                                Margin = new MarginPadding { Bottom = 10 },
+                                SelectorActive = { BindTarget = SelectorActive },
+                            },
+                            privateChannelFlow = new ChannelListItemFlow("DIRECT MESSAGES"),
+                        },
+                    },
+                },
+            };
+        }
+
+        public void AddChannel(Channel channel)
+        {
+            if (channelMap.ContainsKey(channel))
+                return;
+
+            ChannelListItem item = new ChannelListItem(channel);
+            item.OnRequestSelect += channel => this.OnRequestSelect?.Invoke(channel);
+            item.OnRequestLeave += channel => this.OnRequestLeave?.Invoke(channel);
+
+            ChannelListItemFlow flow = getFlowForChannel(channel);
+            channelMap.Add(channel, item);
+            flow.Add(item);
+        }
+
+        public void RemoveChannel(Channel channel)
+        {
+            if (!channelMap.ContainsKey(channel))
+                return;
+
+            ChannelListItem item = channelMap[channel];
+            ChannelListItemFlow flow = getFlowForChannel(channel);
+
+            channelMap.Remove(channel);
+            flow.Remove(item);
+            item.Expire();
+        }
+
+        public ChannelListItem GetItem(Channel channel)
+        {
+            if (!channelMap.ContainsKey(channel))
+                throw new ArgumentOutOfRangeException();
+
+            return channelMap[channel];
+        }
+
+        private ChannelListItemFlow getFlowForChannel(Channel channel)
+        {
+            switch (channel.Type)
+            {
+                case ChannelType.Public:
+                    return publicChannelFlow;
+
+                case ChannelType.PM:
+                    return privateChannelFlow;
+
+                default:
+                    throw new ArgumentOutOfRangeException();
+            }
+        }
+
+        private class ChannelListItemFlow : FillFlowContainer
+        {
+            public ChannelListItemFlow(string label)
+            {
+                Direction = FillDirection.Vertical;
+                RelativeSizeAxes = Axes.X;
+                AutoSizeAxes = Axes.Y;
+
+                Add(new OsuSpriteText
+                {
+                    Text = label,
+                    Margin = new MarginPadding { Left = 18, Bottom = 5 },
+                    Font = OsuFont.Torus.With(size: 12),
+                });
+            }
+        }
+
+        private class ChannelListScrollContainer : OsuScrollContainer
+        {
+            protected override bool OnHover(HoverEvent e)
+            {
+                ScrollbarVisible = true;
+                return base.OnHover(e);
+            }
+
+            protected override void OnHoverLost(HoverLostEvent e)
+            {
+                ScrollbarVisible = false;
+                base.OnHoverLost(e);
+            }
+
+            protected override ScrollbarContainer CreateScrollbar(Direction direction)
+                => new ChannelListScrollBar(direction);
+
+            protected class ChannelListScrollBar : OsuScrollbar
+            {
+                private const float BAR_SIZE = 4;
+                private const float BAR_MARGIN = 7;
+
+                public ChannelListScrollBar(Direction scrollDir) : base(scrollDir)
+                {
+                    Size = new Vector2(BAR_SIZE);
+                    Margin = new MarginPadding { Left = BAR_MARGIN };
+                    CornerRadius = 2;
+                }
+
+                public override void ResizeTo(float val, int duration = 0, Easing easing = Easing.None)
+                {
+                    Vector2 size = new Vector2(BAR_SIZE, val);
+                    this.ResizeTo(size, duration, easing);
+                }
+            }
+        }
+    }
+}