Source Editor: introduce the extensions registry

At the moment, there's no control over extensions in Editor Lite so if we create several instances of the editor on the same page with the same set of extensions, they most probably will be fetched for each particular instance. Also if we apply an extension to a file and then remove that extension (for example re-naming the file), when we need to apply the same extension again (re-name back to the original extension), we will have to fetch it again.

Also, the registry would allow to keep the extensions' settings scopped and would not polite the global scope of the instance.