actionmuxer: Correctly notify actions after reparenting

actionmuxer: Correctly notify actions after reparenting

When registering an observer, we send a notification and for that we need
to query the action's state and param type. When setting up a muxer parent,
same thing happens, except the action is queried on the parent instead.

This means that the muxer will notify observers about the parent's actions,
but not about its own.

Add a test to verify it works.

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/5861