Feature/css

merged again from sebaslv/related-docs and made needed changes.
@sebaslv, you can take a look as well.
You're right, liveDocument currently just triggers this event when it receives the Document.Related message from the protocol handler, but I wanted to keep the structure in place.  This is similar to _onConnect in LiveDocument.js.
I've not triggered the event on stylesheet added & removed cases yet, since I don't have a test right now to dynamically add/remove stylesheets, but it needs to be done later following the same logic.