widget: Fix set_focus_child handling during focus changes

widget: Fix set_focus_child handling during focus changes

10b5ec20 made sure not to set focus_child to NULL all the way up to the
top, but only up to the common ancestor. However, it would never set it
on the common ancestor itself, which would therefore remain with a
focus_child set when it shouldn't.

A manifestation of the bug: focus column headers of a treeview, press Tab.
Now pressing Shift+Tab will go to another widget and not the column
headers, and Tab will (appear to) do nothing, all because the treeview
still has a focus_child set to column headers after a grab_focus().

Signed-off-by: Olivier Brunel <jjk@jjacky.com>
https://bugzilla.gnome.org/show_bug.cgi?id=723402