tsdemux: disentangle requested program number from active program number

tsdemux: disentangle requested program number from active program number

The program_number attribute was overloaded, trying to indicate both
the currently playing program, and the program requested via the
"program-number" property.  The end result was that setting the
property didn't work (see #690934).

I added a new requested_program_number field rather than reviving the
current_program_number field because it seemed this would result in
fewer changes overall and be less confusing.  It breaks symmetry with
the "program-number" property, but it retains parallels with the likes
of program->program_number.

Because gst_ts_demux_reset is called after the properties have been
parsed, requested_program_number is initialised in gst_ts_demux_init.
Whether this is exactly the right place, I don't know.