This adds the mechanisms in gnome-shell 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 gnome-shell part implements the UI bits, like the timeout notifications using a pie-timer and the dwell-click selection using a drop-down menu.
This requires mutter!512 (merged)