Refactor common demux code structures into utility functions.
See the complete list below:
- rearrange arguments of dvbpsi_demux_new_cb_t callback
- define dvbpsi_demux_gather_cb_t callback type
- define dvbpsi_demux_detach_cb_t callback type
- new utility function dvbpsi_NewDemuxSubDecoder()
- new utility function dvbpsi_DeleteDemuxSubDecoder()
- new utility funciton dvbpsi_AttachDemuxSubDecoder()
- new utility function dvbpsi_DetachDemuxSubDecoder()