gtk: Use gtk generic event signal instead of motion-notify-event

The GDK motion-notify-event isn't generated when the pointer goes out
of the target window even if the pointer is grabbed, which essentially
means to lose the pointer tracking in gtk-ui.

Meanwhile the generic "event" signal is sent when the pointer is
grabbed, so we can use this and pick the motion notify events manually
there instead.

Reference: https://bugzilla.novell.com/show_bug.cgi?id=849587
Tested-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>
Tested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
