The signals for the action group were being disconnected when the action
group was explicitly removed from the GActionMuxer but the same was not
being done when it was finalized.
This means that a change in the state of an action group that used to be
associated with a finalized GActionMuxer would result in a crash. This
would happen for stateful application actions after closing a window.
These were destined for GLib, but they don't really make sense as a
public API. Instead, we'll copy/paste them around between the various
codebases that need to render menus.
