gtk: check for existing grabs in gd_grab_{pointer,keyboard}

If a grab is already active for our window, do nothing.
If a grab is already active for another window, release it.

Cleanup some checks and ungrab calls in the code which are
not needed any more.

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