Add API to inhibit unfocus of the cursor surface/actor
Add API to ClutterSeat that allows inhibiting the unsetting of the pointer focus surface. This can be useful for drawing custom cursor textures like the magnifier of gnome-shell does.
In the future this API should also control unsetting of Clutters focus-actor, not just the focus surface, that's not really needed right now since we never unset the focus-actor anyway.
Supersedes !1046 (closed)