Allow persistent state to be supplied to an object on its creation.
Re-add id-generation functionality to BlockSet.

This wa lost when it stopped inheriting from Block.
Remove no-longer-needed special case for SpectrumView ID.
Fix assumption that all monitor widgets are of the RF spectrum.
Explicitly compute the filter for rational_resampler in make_resampler.

This is a workaround for bug <http://gnuradio.org/redmine/issues/745>,
but it also allows us to share some of the design calculations between
the rational_resampler and pfb implementations for consistency.