gst/equalizer/gstiirequalizer.c: Replace filters with a bit better filters for which we can actually find documentati...

Original commit message from CVS:
* gst/equalizer/gstiirequalizer.c:
(gst_iir_equalizer_band_set_property),
(gst_iir_equalizer_band_get_property),
(gst_iir_equalizer_band_class_init), (arg_to_scale),
(setup_filter), (gst_iir_equalizer_compute_frequencies):
Replace filters with a bit better filters for which we can actually
find documentation, which don't change anything on zero gain, etc.
Make the frequency property of the bands writable, rename the
band-width property to bandwidth and change the	meaning to the
frequency difference between bandedges, change the meaning of the
gain property to dB instead of a weird scale between -1	and 1 that
has no real meaning.