clutter: Move pointer a11y settings management from MetaInputSettings
All pointer a11y is a fabrication of Clutter backend-independent code, with the help of a ClutterVirtualInputDevice and with some UI on top.
On the other hand, MetaInputSettings is a backend implementation detail, this has 2 gotchas:
- In the native backend, the MetaInputSettings (and pointer a11y with it) are initialized early, before the ClutterSeat core pointer is set up.
- Doing this from the MetaInputSettings also means another dubious access from the input thread into main thread territory.
Move the pointer a11y into ClutterSettings, making this effectively backend-independent business, invariably done from the main thread and ensured to happen after seat initialization.
Fixes: #1765 (closed)