Implement ChannelControlItem for new chat design

Adds new component `ChannelControlItem` and it's child components to be
used as the clickable control in the new chat sidebar for joined
channels.

Has public properties `HasUnread` and `MentionCount` to control the
display of the channel having unread messages or mentions of the user.

Channel select/join requests are exposed via `OnRequestSelect` and
`OnRequestLeave` events respectively which should be handled by a parent
component.

Requires a cached `Bindable<Channel>` instance to be managed by a parent
component.

Requires a cached `OveralayColourScheme` instance to be provided by a
parent component.

osu.Game/Overlays/Chat/ChannelControl/ControlItemMention.cs

@@ -0,0 +1,77 @@
+// 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.Graphics;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Shapes;
+using osu.Game.Graphics;
+using osu.Game.Graphics.Sprites;
+using osuTK;
+
+namespace osu.Game.Overlays.Chat.ChannelControl
+{
+    public class ControlItemMention : CircularContainer
+    {
+        private int mentionCount = 0;
+        public int MentionCount
+        {
+            get => mentionCount;
+            set
+            {
+                if (value == mentionCount)
+                    return;
+
+                mentionCount = value;
+                updateText();
+            }
+        }
+
+        private OsuSpriteText? countText;
+
+        [Resolved]
+        private OverlayColourProvider? colourProvider { get; set; }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            Masking = true;
+            Size = new Vector2(20, 12);
+            Alpha = 0f;
+
+            Children = new Drawable[]
+            {
+                new Box
+                {
+                    RelativeSizeAxes = Axes.Both,
+                    Colour = colourProvider!.Colour1,
+                },
+                countText = new OsuSpriteText
+                {
+                    Anchor = Anchor.Centre,
+                    Origin = Anchor.Centre,
+                    Font = OsuFont.Torus.With(size: 11, weight: FontWeight.Bold),
+                    Margin = new MarginPadding { Bottom = 1 },
+                    Colour = colourProvider.Background5,
+                },
+            };
+
+            updateText();
+        }
+
+        private void updateText()
+        {
+            if (mentionCount > 99)
+                countText!.Text = "99+";
+            else
+                countText!.Text = mentionCount.ToString();
+
+            if (mentionCount > 0)
+                this.Show();
+            else
+                this.Hide();
+        }
+    }
+}