Cleaned up playlist multilevel sorting algorithm. Added album artist special case to playlist sorting.

It fixes an issue with two albums being merged on the playlist when they happen to have the same name, and sorting by album is enabled.
Originally thought to fix bug 271105, it actually fixes some other, but connected, one (see my comment https://bugs.kde.org/show_bug.cgi?id=271105#c10 ).

Thanks go to Konrad Zemek for the patch.

REVIEW: 110070