TrackScheduler, ClientPlayer bugs

[Mellich]

There is a bug in the track scheduler or the client player, that makes the playback impossible. After skipping a track, the player doesn't react to new prompts from the server (except seeking)

[Mellich]

double skipping through the client now fixed.
Skipping not safe, yet: sometimes the playthread does not terminate (maybe he doesn't receive the signal from vlc)

[Mellich]

skipping now fixed. Remaining problem: sometimes the player does not react to play/pause notifications properly (sometimes after the third press).

[Mellich]

changing deadline of this issue to Milestone 1, because it not really has something to do with the new features and is currently only a stability problem on windows systems.