core: Add support for int64 values

https://bugzilla.gnome.org/show_bug.cgi?id=740761
core: Correct value type for file sizes

An int is obviously too small a value to fit a filesize in bytes, with
its maximum at 2GB. Use an int64 instead.

Note, this breaks ABI but should not break API.

https://bugzilla.gnome.org/show_bug.cgi?id=740761
core: add GRL_METADATA_KEY_MB_TRACK_ID

MusicBrainz track identifier.

https://bugzilla.gnome.org/show_bug.cgi?id=732878
core: add GRL_METADATA_KEY_MB_ARTIST_ID

MusicBrainz artist identifier.

https://bugzilla.gnome.org/show_bug.cgi?id=732878