[Overview] Delay repositioning after closing windows

When closing windows in the window picker, the remaining windows
move to their new positions, which prevents the user from performing
additional actions in the picker until the animation stops and the
user adjusts to the new layout.

Instead we try to be smart and delay the repositioning, either until
the pointer stops moving or until it leaves the workspace.

https://bugzilla.gnome.org/show_bug.cgi?id=611167