mpegtsmux: Add SCTE-35 support

mpegtsmux: Add SCTE-35 support

This adds two properties:
* scte-35-pid: If not 0, enables the SCTE-35 support for the current
  program. This will write the proper PMT and send SCTE-35 NULL
  commands (i.e. heartbeats) at a regular interval
* scte-35-null-interval: This specifies the interval at which the
  NULL commands should be sent

Sending SCTE-35 commands is done by creating the appropriate SCTE-35
GstMpegtsSection and then sending them on the muxer. See the
associated example