Improve track sequencing by removing the track sequencing classes.
This removes one of my first contributions to JuK :( But it's worth it because the extra code is not worth the complexity, seeing as how the job is really pretty simple in the first place, even with album random play and randomized playback. I believe this also fixes some bugs, including some longstanding ones. Bug 417551 (being unable to drag and drop into Play Queue) had some related work in a recent commit but was still broken until now. BUG:100356 BUG:166711 BUG:302250 BUG:303901 BUG:336637 BUG:353259 BUG:404157 BUG:417551 FIXED-IN:21.04