Drag up/down to set arbitrary FM demod bandwidth

Shouldn't need separate WBFM/FM/NFM setting this way -- Just "FM" and
then set arbitrary bandwidth by dragging.

Also removed redundant demod resampling stages left over from early
experiments.
Proper demodulator resamp and filter reinit
Disable mouse locking on OSX -- causes event loop
Removed unused/empty source files
Demodulator can now tune offset!

Demodulator will up-convert / down-convert by mixing VCO with incoming
IQ to shift frequency -- No longer limited to center freq. can listen
anywhere on the waterfall.
Merge pull request #11 from cjcliffe/canvas_interaction

Canvas interaction tuning + bandwidth, demodulator refactor, demod offset tuning