This allows removing it from `class_db.h`, significantly reducing the amount of files
that include it transitively.
Also includes some include cleanup in `control.h` and `rich_text_label.h` done while
ensure they don't depend on `callable_mp`.
This does not apply to text printed to the Output panel, as the fixed-width
font may not have the glyph in its character set (or if it does, the arrow
character may be hard to read).
This PR adds 3 types of audio streams used for interactive music support.
* AudioStreamInteractive: Allows setting several sub-streams and transition between them with many options.
* AudioStreamPlaylist: Allows sequential or shuffled playback of a list of streams.
* AudioStreamSynchronized: Allows synchronous playback of several streams, the volume of each can be controlled.
Theese three stream types can be combined to create complex, layered interactive music and transitions between them, similar to software such as WWise.