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/ChannelListSelector.cs

@@ -0,0 +1,84 @@
+// 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 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;
+
+namespace osu.Game.Overlays.Chat.ChannelList
+{
+    public class ChannelListSelector : OsuClickableContainer
+    {
+        public readonly BindableBool SelectorActive = new BindableBool();
+
+        private Box hoverBox = null!;
+        private Box selectBox = null!;
+
+        [BackgroundDependencyLoader]
+        private void load(OverlayColourProvider colourProvider)
+        {
+            Height = 30;
+            RelativeSizeAxes = Axes.X;
+
+            Children = new Drawable[]
+            {
+                hoverBox = new Box
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Colour = colourProvider.Background3,
+                    Alpha = 0f,
+                },
+                selectBox = new Box
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Colour = colourProvider.Background4,
+                    Alpha = 0f,
+                },
+                new Container
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Padding = new MarginPadding { Left = 18, Right = 10 },
+                    Child = new OsuSpriteText
+                    {
+                        Anchor = Anchor.CentreLeft,
+                        Origin = Anchor.CentreLeft,
+                        Text = "Add More Channels",
+                        Font = OsuFont.Torus.With(size: 17, weight: FontWeight.SemiBold),
+                        Colour = colourProvider.Light3,
+                        Margin = new MarginPadding { Bottom = 2 },
+                        RelativeSizeAxes = Axes.X,
+                        Truncate = true,
+                    },
+                },
+            };
+        }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+
+            SelectorActive.BindValueChanged(selected => selectBox.FadeTo(selected.NewValue ? 1 : 0));
+            Action = () => SelectorActive.Value = true;
+        }
+
+        protected override bool OnHover(HoverEvent e)
+        {
+            hoverBox.FadeIn(300, Easing.OutQuint);
+            return base.OnHover(e);
+        }
+
+        protected override void OnHoverLost(HoverLostEvent e)
+        {
+            hoverBox.FadeOut(200, Easing.OutQuint);
+            base.OnHoverLost(e);
+        }
+    }
+}