PlayerManager: Queue up next tracks instead of stop/play cycle.

I did this because I got annoyed about my own log messages about
inhibiting/uninhibiting power management.

This ultimately was because Phonon forces a stop and then restart
sequence whenever forcibly changing the current media source, as we were
doing to switch tracks.

Phonon provides a way to queue up additional tracks to play when
playback is about to end which keeps itself in a playing state. I use
this method, which may also theoretically make gapless playback work.