atspi: Properly filter out parent actions

atspi: Properly filter out parent actions

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.