Timeshift improvements

Please read the individual commit messages for the details (not just the first line).

A few notes:

* STimeshiftStatus uses a bool instead of an int since this is not C from 1995
* It seems no one else implements PauseStream(), though I don't see why not
* The SendSpeed() method is a bit funny since it assumes the speed parameter is in XBMC units, not tvheadend units. I think overall it would be best to factor out such differences to client.cpp so sanity and readability can be preserved in the inner layers (like I've done with GetPlayingTime()).