Allow files to be added to a media

In order for VLC to add new files it detects, we need to expose Media::addFile outside of the media library.

AFAICS, we need:

• A public addFile method, which takes an mrl and a file type
• A public removeFile method, which takes an mrl (file type probably isn't required), or maybe an id, or both variants could be exposed.

A few open questions need to be answered:

• Are those files expected to be monitored by the media library
• If so, should it be only for the files that are found in folders monitored by the media library