Data is copied a lot

[hansvi]

SpectrogramPlot::getLine copies the sample data to a buffer, then this is copied to an fftw aligned buffer in fft.cpp, the fft is calculated, and the result copied back to another buffer. That's a lot of data being copied unnecessarily. It might be better to let the FFT class calculate the logPower directly from the sample source? this would group the calculations, making it easier to optimize? Maybe there are even better ways? I can implement this if you give the OK.