MPMusic: Sorting by MusicTrackNumber now reflects disc number

Similarly as with TVEpisodeNumber where sorting has to count also with seasons, MusicTrackNumber so far did not take into account that multi-disc albums, when properly tagged (DISCNUMBER ID3 tag), require to be sorted with respect to the disc number. This can be accomplished by sorting the query output from MP MusicDatabase based on iDisc value first. 

Tested & works fine