Refactor sync handling

This factors out the state tracking to a separate class. The class performs locking internally so the caller doesn't need to bother with it.

The second commit adds calls to WaitForState() to the various getters XBMC calls during initialization. This default timeout is taken from the response timeout setting. This should prevent XBMC from fetching channels (and other things) before the client has actually loaded them all, which would otherwise cause missing channels.

@adamsutton the implementation of WaitForState() isn't exactly pretty but it seems to do the job. I have not tested this on Windows yet, could be that we need to include a platform header which defines usleep().