If a muxer does not contain an action group with the given prefix, chain
up to the "parent" muxer to look for it.
This initial implementation is rather inefficient. It will lead to
changes on action groups associated with parent muxers being broadcast
to all children (regardless of if anybody there is interested or not).
An optimised version will follow soon.
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.
