Expose modulation type in gui, support changing modulation type

[wizardishungry]

see https://github.com/sysrun/rtl-sdr

[davephillips]

Ha, I was just going to suggest this feature. :) An AM/FM switch would be sweet, most of the AM stations here are hellfire & brimstone nutcase broadcasts, great material for processing through the Rack. Btw, I'm having a great time with this whole rtl_sdr thing, thanks so much for the inspiration.