File input fixes and enhancements
Corrected the wrong times display on large files due to counter overflow. This should fix issue #206. A playback accelerator has been added to allow waterfall fast forward helping getting faster to the point of interest.
Spectrum: added max mode
Added a maximum mode in the list of averaging modes. This does not perform an average actually but takes the maximum over the same number of consecutive FFTs. This can help spotting transient bursts occurring during the "averaging" period. This implements issue #207