Added discnumber & composer to the DB schema and sorting by discnumber.

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.

data/sql/dbmigrate-26_to_27.sql

@@ -1,4 +1,4 @@
--- Script to migate from db version 6 to 27
+-- Script to migate from db version 26 to 27
 -- Nothing to do
 
 CREATE TABLE IF NOT EXISTS collection_attributes (

data/sql/dbmigrate-27_to_28.sql

@@ -0,0 +1,7 @@
+-- Script to migate from db version 27 to 28.
+-- Added albumartist and discnumber to file_join
+
+ALTER TABLE file_join ADD COLUMN composer INTEGER REFERENCES artist(id) ON DELETE CASCADE ON UPDATE CASCADE DEFERRABLE INITIALLY DEFERRED;
+ALTER TABLE file_join ADD COLUMN discnumber INTEGER;
+
+UPDATE settings SET v = '28' WHERE k == 'schema_version';