clutter/stage: Repick when pointer actor goes unmapped

I've overseen quite an important case in commit 98a5cb37: Repicking only when actors get destroyed is not enough, we actually need to repick when actors go hidden/unmapped.

While we could also listen to notify::mapped just like we listen to notify::reactive, it seems better to avoid using property notifications here due to the usage of g_object_freeze/thaw_notify() in ClutterActor. It can lead to the stage receiving a notify::mapped with mapped = true for a pointer actor, which really shouldn't happen (just like notify::reactive with reactive = true shouldn't happen).

Fixes: gnome-shell#5124 (closed)