fix transit to next track & abouttofinish madness

- rename checkgaplesswaiting to hasnexttrack
- when the player emits endedstate and we have a next track -> continue,
  otherwise emit abouttofinish and stop
- don't discard state changes when we have pending tracks
- when setNextSource is called when in stopped state, make sure we
  go to the source right away as that would be caused by a timing issue
  WRT queuing of signals, so libphonon wanted us to play that song before
  the other finished, but due to timing we only get the song after we are
  stopped

before putting this into 0.6 I'd like to have some statements that this
does not break any existing software! in fact I'd rather not put this in
0.6 at all...

CCMAIL: kde-multimedia@kde.org

REVIEW: 107118