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 will allow decoupling `display_server.h` from a number of headers in the
codebase which only require those enums and not all the DisplayServer API.