[dvbviewer] make RTSP working

Note: RTSP is currently not compatible with timeshift
[dvbviewer] force channel sync on start

Users might switch between using RTSP/timeshift which requires new
stream urls. Changing these settings already requires a PVR restart but
this won't trigger a sync. So we now force this
[dvbviewer] fix channel index in GenerateTimer
[dvbviewer] use c_str() just to make sure
[dvbviewer] avoid double encodes of format specifiers