QAccessible::ActionChanged event not sent anywhere in Qt

The QAccessible::ActionChanged event should presumably be sent when the actions of an a11y element changes, based on http://accessibility.linuxfoundation.org/a11yspecs/ia2/docs/html/_accessible_event_i_d_8idl.html.

We don't seem to do this anywhere in Qt, nor respond to the event in the a11y backends. Quick attempts at plumbing this on macOS failed, as there doesn't seem to be a way to report action changes to the system a11y.