User selectable FFT size proposal

Hi Al,
 I find useful to have an option to select size of used FFT (i.e. NFFT) as sometimes I want to see more detailed view of a particular signal spectrum. Therefore I've made an extension that allows user selection of FFT size. I'd be glad if You consider reviewing this change and including in master branch in case You find it useful and desirable.

Besides I've made a small fix to call self.set_controls(True) when file is opened, so that user can perform export of loaded data to csv.

Brux