listbase: Use set_focus_child properly

The EventControllerFocus on the list item, updates the list base focus
tracker and scrolled to position any time the list item enters focus.
This works when interacting within a single window, but has unexpected
results when changing focus between multiple windows.

Instead of using the focus controller workaround, just make
gtk_list_base_update_focus_tracker the set_focus_child vfunc