Modify client spectrum widgets to support real FFTs.

Not quite complete: the spectrum plot is still doing wrap-around.
When the source type is not IQ, do a real FFT.

Or rather, complex FFT and throw away half of it, because I tried
logpwrfft_f and it just produces the two-sided output anyway.
Remove Hilbert filter from AudioDevice.

We now have the infrastructure in place to display the real spectrum
without a redundant half, which gives the UI we want without requiring
an extremely sharp filter.