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.
Bug 841977 - Part 3: added marionette tests
Bug 796305 - Part 3: Marionette test for single disc
Merge pull request #23961 from hfiguiere/disc-numbers-list

Track sorting and listing improvements. (Bug 796305 and bug 841977) r=squib