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

servers/display/display_server.cpp

@@ -1706,6 +1706,7 @@ void DisplayServer::_bind_methods() {
 	BIND_ENUM_CONSTANT(ROLE_TITLE_BAR);
 	BIND_ENUM_CONSTANT(ROLE_DIALOG);
 	BIND_ENUM_CONSTANT(ROLE_TOOLTIP);
+	BIND_ENUM_CONSTANT(ROLE_REGION);
 
 	BIND_ENUM_CONSTANT(POPUP_MENU);
 	BIND_ENUM_CONSTANT(POPUP_LIST);

servers/display/display_server.h

@@ -606,6 +606,7 @@ public:
 		ROLE_TITLE_BAR,
 		ROLE_DIALOG,
 		ROLE_TOOLTIP,
+		ROLE_REGION,
 	};
 
 	enum AccessibilityPopupType {