Rename our internal GActionMuxer, GActionObserver and GActionObservable
classes and interfaces to have names in our own namespace.
These classes were originally intended for GIO but turned out to be too
special-purpose to be useful there, so we never made them public API but
have just been copying them around (without bothering to properly rename
them). Now that other people will be copying them out of Gtk, it's even
more important to prevent this namespace abuse from spreading further.
