Provide an implementation of the fileapi for oc6 build on top of the old api

This provides an implementation of the upcoming fileapi (see the `fileapi` branch) for oc6.

It's build as a fairly small layer on top of the current api, in contrary to the `fileapi` branch which is the other way around.

This implementation of the api doesn't bring the advantages that the "proper" implementation has but provides a stable implementation for oc6 to be used in the public api for files.

Both implementations are api compatible so once the "proper" implementation is done it can function as a drop in replacement.

For using the api, OC provides access (through a DI container or whatever) to the `Root` node which functions as entry point for the api.

cc @DeepDiver1975 @karlitschek @tanghus