text-input: Add v3 of the text-input protocol

This new protocol description is an evolution of v2.

- All pre-edit text styling is gone.
- Pre-edit cursor can span characters.
- No events regarding input panel (OSK) state nor covered rectangle.
  Compositors are still free to handle situations where the keyboard
  focus rectangle is covered by the input panel.
- No set_preferred_language request for clients.
- There is no event to send keysyms. Compositors can use wl_keyboard
  interface instead.
- All state is double-buffered, with specified defaults.
- The compositor can be notified about external changes to the state.
- The client can detect outdated requests.

Signed-off-by: Dorota Czaplejewicz <dorota.czaplejewicz@puri.sm>
Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>