gtk: don't grab input when entering fullscreen.

gtk: don't grab input when entering fullscreen.

Kick off all grabbing logic from fullscreen mode.  In the current state
it seems to create more problems than it solves.  Try running qemu/gtk
fullscreen on one head of a multihead host for example ...

There probably was a reason the grab-on-fullscreen logic was added in
the first place.  So please test and report any issues so we can try to
find a sane way to handle it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>