keyboard: Schedule widget redraw instead of gdk_window_invalidate_rect
The later would invaliate the drawn area again and again in a ::draw()
handler. This caused infinite redraws when e.g. a key was locked. To
reproduce break e.g. on eek_gtk_keyboard_real_draw()
and see how it is
invoked again and again.
This kept the keyboard and the compositor quite busy. @raisins it'd be cool to have your input here.