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 :) :)