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.