Refactor FM demodulator to use lower rates and fewer filters.

There is no longer a "post_demod_rate", and only one audio filter.

This makes NFM slightly better, and WFM slightly worse (because it lost
early decimation).
Use a lower intermediate rate in WFM demodulation.
Remove demodulator rebuild from WFM stereo switch.

The bug has been troubleshot ( http://gnuradio.org/redmine/issues/713 )
and worked around in the resampler/filter tools, so we don't need this
workaround.