gizmo: Fall back to widget focus functions

This fixes keyboard tab navigation inside HdyWindows.

This is a delta with GtkGizmo, which has no focus fallback. But Gtk also has access to the private gtk_widget_focus_self and gtk_widget_focus_child functions, which it uses in the few places it cares about gizmo focus.

Handy never overrides the focus, and it probably doesn't care about disabling focus, right? I assume the default widget focus handling is mostly right.