This can happen when starting Ardour with the Preference pane
as initial tab. The editor, mixer are not realized, so
-propagation fails with:
IA__gtk_widget_event: assertion 'WIDGET_REALIZED_FOR_EVENT (widget, event)' failed
We now check from the focus widget (if any) for any widget heirarchy bindings, and try to use them.
Next use the "top level" bindings passed in (top level is quoted because they may be owned by a tab,
rather than a window).
Finally, if the event is still not handled, try the global bindings
This model more closely matches what I think a reasonable programmer with experience of other
GUI toolkits would expect, and allows us to have multiple bindings present (though not
simultaneously used) in a given window
