Add compilation support to exaile.

Compilation albums should be sorted into a 'Various Artists' category in the treeview.

The main issue lies in detecting compilation albums. There are two ways to do this: detection from 'album artist' tags, and detection based on album tag and filesystem layout.

'album artist' method:
    freeform tag formats (ogg, apev2) just use "albumartist" for this
    mp3 does not define a tag for this
        itunes/WMP use TPE2
        foobar2000 uses TXXX:AlbumArtist
        - we should support both of these as toggleable options in the preferences
    add other formats here (mp4? asf?)

filesystem layout method:
    if a set of tracks have the same album and exist in the same directory, assume that they are a compilation album and treat them as such
    - this should be a toggleable option in the preferences, since not all users may like such heuristics or have collection layouts conducive to them.