This came up while testing the new realm thread, where `MusicController`
would fall over when `OsuTestScene` changes the global beatmap from an
async load thread (causing a cross-thread realm access).
We don't want to have to schedule every usage of these bindables, so
this seems like a good constraint to put in place.
This paves the way for the framework code quality change
(https://github.com/ppy/osu-framework/pull/4873) which stops exposing
the constructor.
Most of the restructuring here is required to give `PreviewTrackManager`
an adjustable target to apply the global mute.
Changes all usages of `KeyCombination.ReadableString()` to
`ReadableKeyCombinationProvider.GetReadableString()`.
Subscribing to `KeymapChanged` is only required in `KeyButton`.
All other places query `GetReadableString()` every time.
Until now, the `RealmContextFactory` would be disposed as part of the
drawable hierarchy. This is too early, as it may be being used by higher
level components (like `ConfigManager`s, see #15115) that perform final
operations after the drawables have been disposed.
Seems to make sense moving this out of the drawable hierarchy and in
line with how we were doing things with EF.