Execute MarkWatched in another thread.

Continuation of pull request #644.

* MarkWatched is now executed in another thread
* Harmonisation of the behavior of mark as watched :
  When marking a video as watched in Music mode, xbmc will not select
  the next entry, but it was the case for Video mode.
* Now MarkWatched works, even when scanning

Sorry for another pull request for this feature, I have wipe my old fork, in order to suppress the bad merges from xbmc/master