GUI: Add accessibility region role for landmark navigation

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

scene/gui/tab_container.cpp

@@ -565,6 +565,7 @@ void TabContainer::_on_tab_hovered(int p_tab) {
 void TabContainer::_on_tab_changed(int p_tab) {
 	callable_mp(this, &TabContainer::_repaint).call_deferred();
 	queue_redraw();
+	queue_accessibility_update();
 
 	emit_signal(SNAME("tab_changed"), p_tab);
 }