Fix set_focus_child handling during focus changes

Fix set_focus_child handling during focus changes

In many cases we used to set focus_child to NULL all the way up
to the top and then to the right value, even if there was
a common ancestor, meaning these see a temporary NULL value for
focus_child. Only when the new focus widgets direct parent was
in the previous ancestor list did we stop early.

This fixes that by always stopping propagation when reaching
the common ancestor.