Chrono: add method SignedSongTime::Negative()
Chrono: add methods ToS(), RoundS()
Chrono: add SignedSongTime::FromS(unsigned)
Chrono: convert SongTime to SignedSongTime implicitly
Tag: use SignedSongTime for the song duration
DecoderControl: use SignedSongTime for the song duration
PlayerControl: use SignedSongTime for the song duration
PlayerThread: use SongTime for elapsed_time
MultipleOutputs: use SignedSongTime for elapsed_time
MusicChunk: use SignedSongTime for the time stamp