Peak detection

This branch adds support for peak detection and "snapping".
That is if the user clicks near a peak the frequency is automatically set to the peak.
Peak detection is done by comparing the value of the FFT transform (actually the value to be drawn) to the mean and the standard deviation of the data. The distance of a peak from the mean must be of at least "c" times the standard deviation. This can be configured using the method setPeakDetection(bool enabled, double c).

There is a checkbox in the fft transform settings to enable/disable peak detection with c=2.

Example: ![Example](http://i.imgur.com/SpfmZgq.png)