Add support for nesting embeddables

This is a possible approach towards adding support for nesting embeddables (embeddables inside embeddables). I'm not sure whether this implementation is the best solution but I think it is something to start with. It seems to work flawlessly so far but I'm not sure if I missed something, so any feedback is welcome ;)
A good way to solve this is to keep an internal reference map (new property in the CMF) somehow. Finding loops would be easy then.