query filecache for media files, overwrite path with path from filecache

@MorrisJobke @DeepDiver1975 Here is something to work with. This PR uses the filecache to search for audio files and overwrites the path of the track entities with the one from the filecache. A similar hack would be needed for cover art, but I wanted to get this out ASAP for discussion.

Personally, I think this is an ugly solution ... then again the sharing implementation has its deficiencies. The whole reason we have to go this route is because sharing allows sharing a folder with a single share entry, not marking all affected files as shared ... hm ... need to chew on that ...