window: Stop duplicating focus change events

window: Stop duplicating focus change events

We had code in gtkwindow.c that generated duplicate,
and defective, focus-change events, in the following
way:
- gtkmain.c generates a chain of focus-change events
  for moving focus from one window to another
- gtkwindow.c catches a focus-in event in the middle
  of this chain and sets itself as 'active'
- and then it proceeds to generate focus-change
  events towards its own focus widget without a
  related target
This is not necessary since we gtkmain.c already
generates a complete sequence of focus-change events.
So stop doing it.