canvas-container: rework canvas item focusing

 - Currently, atk_focus_tracker_notify() is used, which is deprecated as
   of ATK 2.9.4. Using atk_object_notify_state_change() with
   ATK_STATE_FOCUSED is recommended. The deprecated call was replaced.

 - For every selection, the previous one should get unfocused.
   Currently, only the keyboard focus is tracked. That was extended to
   mouse focus with a flag for keyboard focus for indicator rendering.

https://bugzilla.gnome.org/show_bug.cgi?id=762236