make \OC\Files\Node\Node implement the FileInfo interface

Instead of extending I opted for composition which allows lazy loading the fileinfo.

cc @DeepDiver1975 @PVince81 
This will break the public API https://github.com/owncloud/core/blob/master/lib/public/files/file.php#L55

Please just return `$this->getFileInfo()->getMimeType()` or something like that. Maybe also deprecate this method in the public api and point at `getFileInfo`