Add a "pointer to other song" capability

[kevinfrei]

This is basically me being annoyed that I have 12 copies of the same recording of "Smells Like Teen Spirit". If they're the same recording of the same song, I'd like to have some little turd file that indicates that, instead of copying the file around. Something like 'Compilations/VA - 1996 - Eighties Stuff/03 - 80's Band - This File.emp' should contain some json like this:

{
  "original": "../80's Band - 1985 - First Release/05 - This File.flac"
}

with maybe the ability to override stuff in metadata shoved in there for good measure (Album Art might be different, right?)

[kevinfrei]

Started work in the PointerFiles branch

[kevinfrei]

The basic capability is committed in bb61084
Next step is to enable "real" metadata overriding. Currently, the path is used, and then the metadata from the original file.