On map, highlight stations which are in source's band or have receivers.

Support:
* Fixed non-exportedness of source implementations' get_sample_rate.
* VOR map layer no longer draws its own marker.
Add partial workaround/note for VOR layer type error problem.
Give client information about what interfaces a 'block' implements.

This will hopefully be used to reduce the amount of hardcoded parallel
structure in the client (knowing which sub-block widget class to
instantiate) and allow plugins to override them (e.g. a demodulator
with a matching widget).