Save FFT plotter center frequency in settings

[sn4kebite]

Store the selected center frequency in the plotter as fft/fft_center. This will make the plotter center consistent on startup when zoomed in.

[argilo]

Thanks!

This seems to work well, and it fixes what was probably an unintended side effect of #1248 (namely, that the demod frequency would no longer be in view upon startup, because the zoom slider no longer used the demod frequency as its center).

[argilo]

The following undefined behaviour occurs on startup:

/home/argilo/prefix_310/src/gqrx/src/qtgui/plotter.cpp:918:56: runtime error: signed integer overflow: -9223372036854775808 - 1 cannot be represented in type 'long long int'

This looks to be due to an existing bug, where CPlotter::freqFromX performs a division by zero due to m_Size.width() and m_DPR being zero before the UI is rendered. I'll add a fix here.