Added support for three new metadata elements of a track: Album Artist,
Composer and Disc Number. Since these are not supported in ID3v1 which
TagLib uses as common ground between the different metadata formats, I
had to handle the following cases separately: APE, ASF, ID3v1, ID3v2,
MP4 and OGG.
Modified the DB schema to include composer and discnumber. Provided the
dbmigrate file and bumped the schema version.
Added composer and discnumber data to all relevant queries.
Removed a bogus result->setYear from some queries in dbcmd_resolve.cpp.
