gestures: Make claiming button release work

gestures: Make claiming button release work

When claiming a sequence in a gesture signal handler,
the expected result is that GtkGesture::handle-event
returns TRUE, causing the event to not be propagated
further.

This doesn't work for button release events, since
gtk_gesture_handle_event does the following:
  add point
  emit ::update
  remove point
  check claimed status
The ::update signal is where the application code
claims the sequence. But removing the point purges
the sequence from the gestures memory, so checking
the claimed status returns FALSE.

This patch fixes things to behave as expected, by
checking the claimed status before removing the point.