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.
This was in flyte's design (he was following discord) but doesn't feel
too correct when it's not aligned with anything else. We can revisit
this if/when we have better glyphs to represent channel types.
Test failed locally in `TestPublicChannelMention`. This test seems to
specify that the same message may arrive twice with the same ID, so
rather than overthinking this one I propose we just use `FirstOrDefault`.
```csharp
TearDown : System.AggregateException : One or more errors occurred.
(Sequence contains more than one matching element)
----> System.InvalidOperationException : Sequence contains more than
one matching element
--TearDown
at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean
includeTaskCanceledExceptions)
at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout,
CancellationToken cancellationToken)
at osu.Framework.Extensions.TaskExtensions.WaitSafely(Task task)
at osu.Framework.Testing.TestScene.checkForErrors()
--InvalidOperationException
at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source,
Func`2 predicate, Boolean& found)
at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1
source, Func`2 predicate)
at
osu.Game.Overlays.Chat.DrawableChannel.<processMessageHighlighting>b__14_0()
in
/Users/dean/Projects/osu/osu.Game/Overlays/Chat/DrawableChannel.cs:line
102
at osu.Framework.Threading.ScheduledDelegate.RunTaskInternal()
```
