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
Two fixes for text field accessibility:
1. Fix character count being off by one due to trailing newline always
being appended to the last line. Add `is_last_line` parameter to
`accessibility_create_sub_text_edit_elements()` to control this.
2. Link adjacent TextRuns via `previous_on_line`/`next_on_line` so
screen readers can properly navigate lines. Without these links,
AccessKit treats each TextRun as a separate line, causing incorrect
announcements when arrowing through multi-line text.
Nolan Darilek
Pāvels Nadtočajevs
Aaron Franke