Fix scratchy/choppy playback in phonon-gstreamer, refactor playback.
The simplifications needed for PlayerManager have the sideeffect of removing crossfade support for now (but that hasn't worked since KDE 3...). Despite the churn to PlayerManager, the actual problem causing poor playback was that the position slider was syncing its GUI update straight back into PlayerManager::seek. This affected phonon-vlc less for some reason but either way the output is now bearable :) :)
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