mpegtsmux: improve conformance to the standard (and ETSI TR 101 290)
This:
- implements a new "bitrate" property, allowing for CBR streams
- extract a base class out of mpegtsmux
- inherit from it in an atscmux class, that implements ATSC-specific E-AC-3 muxing
- ports basetsmux to aggregator
- makes use of the bitrate (if set) to improve PCR accuracy, and the time of retransmission of (P)SI tables
- does a general cleanup, which while nice means whoever reviews will be better off reviewing commit by commit ;)
- fixes a few details on the way