Bug 796305 - Part 1: Extract disc number from metadata.

Modify id3v2.3, ogg and mp4 test file to include that metadata.
Bug 841977 - Part 1: load the # of tracks and disc from metadata.
Improve metadata unit tests
Address review comments
Bug 796305 - Part 2: sort album sublist by track number.
Bug 841977 - Part 2: Show track number instead of index. Also disc number if relevant.