GtkModelButton is no longer derived from GtkButton,
but can still treat it like a button for the purposes
of having a click action. This lets ATs activate
menu items again.
We only want to show relevant, local actions for
widgets, but _gtk_widget_get_action_muxer() will
return the muxer of a parent widget (all the way
up to the toplevel), if the widget does not have
any actions of its own. To detect this situation,
compare what _gtk_widget_get_action_muxer() returns
for the parent widget, and act accordingly.
