Add an interface for the root folder so apps have an interface to deal with storage

@icewind1991 @LukasReschke @PVince81 @nickvergessen @DeepDiver1975

Nope, Folder is a class and Root needs to inherit some methods. As for the IRootFolder interface: it's currently empty but the Root class implements some methods that are on neither the Emitter nor the folder interface. Now depending on our needs we can then always add those methods to the public namespace afterwards ;)