Allow users to change waterfall ranges

[Retartist]

In other SDR programs like SDR sharp or RTLSDR.com, there's a feture that allowes you to change the db range of both the waterfall and the spectrum, so that if your tuner or antenna has a louder noise floor, you can adjust the zero and max range for the waterfall.

In the image shown the background noise and signals are indistinguishable and it makes setting the desired gain on the HACKrf quite hard.