rx_fft: normalize fft windows

[vladisslav2011]

Always show correct signal magnitude on the panadapter independent of selected FFT window type.

Panadapter levels with this fix applied

Fix #1189

[argilo]

This seems like a good idea.

It looks like the build function has a normalize argument: https://www.gnuradio.org/doc/doxygen/classgr_1_1fft_1_1window.html#a6b4f09baa6c2a7d576dd58ec4ab3f939

Could we use that instead?

[vladisslav2011]

Could we use that instead?

normalize is 3.9+, so not now. Maybe later.

[argilo]

normalize is 3.9+, so not now. Maybe later.

I guess we can't use it at all, because it's for energy normalization. That's fine.

[argilo]

I tested this and it works well. CW peaks now show the correct amplitude regardless of the window.

The amplitude of wideband signals (like the noise floor) varies between window functions because we've chosen to normalize amplitude instead of energy, but I think that's fine.