When chaging to another track, play it from the beginning.

src/playlist.cpp

@@ -84,6 +84,7 @@ bool Playlist::track_next() {
 bool Playlist::set_track(uint8_t track) {
 	if (track < _files.size()) {
 		_current_track = track;
+		_position = 0;
 		return true;
 	}
 	return false;