notebook: Implement MxFocusable
notebook: Implement MxFocusable

Add a simple MxFocusable implementation by forwarding the focus to the
current_page in case it's supported.

clutter-project/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,clutter-project/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,pwithnall/mx