Fixes for UpNext playqueue

This pr addresses some issues reported in #38

The first commit changes the reshuffling logic to only reshuffle the songs that are in the UpNext queue and not the whole playqueue. This should solve the issue 1 in #38 (my guess is that the songs showing up on reshuffling were songs that were still in the queue).

The second commit clears the playqueue on playback stop (abort), so that after adding new songs, the old ones don't show up unexpectedly.

@ejurgensen could you please review the changes? I did only short tests with retune, will hopefully find the time over the week to do some further testing.