Implement ChatTextBox for new chat design

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

Adds new component `ChatTextBox`.
Exposes `BindableBool` `ShowSearch` to change text input behaviour
between normal and search behaviour.

Adds new component `ChatTextBar`.
Exposes `BindableBool` `ShowSearch` which toggles between showing current
chat channel or search icon.
Additionally binds to child `ChatTextBox` components.
Requires a cached `Bindable<Channel>` instance to be managed by a parent
component.

osu.Game/Overlays/Chat/ChatTextBox.cs

@@ -0,0 +1,36 @@
+// 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.Bindables;
+using osu.Game.Graphics.UserInterface;
+
+namespace osu.Game.Overlays.Chat
+{
+    public class ChatTextBox : FocusedTextBox
+    {
+        public readonly BindableBool ShowSearch = new BindableBool();
+
+        public override bool HandleLeftRightArrows => !ShowSearch.Value;
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+
+            ShowSearch.BindValueChanged(change =>
+            {
+                PlaceholderText = change.NewValue ? "type here to search" : "type here";
+                Schedule(() => Text = string.Empty);
+            }, true);
+        }
+
+        protected override void Commit()
+        {
+            if (ShowSearch.Value)
+                return;
+
+            base.Commit();
+        }
+    }
+}