GActionMuxer: add support for parent muxers

GActionMuxer: add support for parent muxers

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.