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`.
Adds ROLE_REGION to allow controls to be marked as accessibility
regions/landmarks.
- Add `accessibility_region` property to Control
- Add ROLE_REGION to DisplayServer and AccessKit mapping
- Prevent Container/ScrollContainer from overriding region role
- Fix TabContainer to update accessibility when tabs change
- Mark editor docks, main screen, bottom panel, and scene tabs as regions
