display/compositor: Emit 'grab-op-end' signal after ungrab happened
We're currently emitting the 'grab-op-end' signal when the grab prerequisites are met, but when display->grab_op is still set to a not-NONE value and thus meta_display_get_grab_op() would return that in the signal callback. And more importantly when this is emitted, devices are still grabbed.
Instead, emit this signal as soon as we've unset all the grab properties and released the devices.
Helps with gnome-shell#1326 (closed)