Return Etag and OC-Fileid after MKCOL call

Moved getEtag() method from File to Node class.
Added override for createCollection() to be able to return the Etag and
OC-Fileid as extra headers after directory creation.

Hopefully the least ugly hack without having to patch SabreDAV upstream.

Please review @DeepDiver1975 @icewind1991 @karlitschek