Mouse accessibility

This adds the mechanisms in mutter to implement mouse accessibility.

Previously, this was handled by a separate daemon (https://wiki.gnome.org/Projects/Mousetweaks) but that cannot work on Wayland (as Wayland clients have no access to the global coordinates and cannot manipulate the pointer).

So this feature needs to migrate to mutter/gnome-shell.

The proposed approach is to implement it in clutter (which handles all events) - That works in X11 as well because mutter has a grab on the pointer in X11 so clutter receives all events.