MetaFile: guess album artist as in SQL collection scanner, compilations

Call albumArtist = ArtistHelper::bestGuessAlbumArtist( albumArtist,
artist, genre, composer ); afer the data are read from file tags.

This makes album artist field consistent for tracks in Local collection
and tracks in file browser or drag & dropped to Amarok playlist, but it
has some downsides:
* Amarok will essentially lie about what album artist is assigned to a
track, most notably will replace empty album artist by track artist
and will replace "Various Artists" by empty artist.
* Editing album artist will be strange in corner cases: in particular
if user sets album artist to "Various Artists", it will be written
to file tags as-is, but displayed as empty album artist. Other case
would be when user sets album artist to empty string, but this is
currently prevented due to a bug/feature in TagDialog.

(every occurrence of "Various Artists" also accepts localized version
of the string)

Above change is needed for upcoming "MemoryCollection: use also album
artist as identifying key in album map" change for UmsCollection, but
if someone opposes, similar change can be made on UmsCollection level.

Also, implement MetaFile::FileAlbum isCompilation() which is now easy
as it can return albumArtist.isEmpty()

DIGEST: unify album artist handling in Amarok