Added basic implementation spectrogram plot

Now the fft is not implemented, a log10 version of the time signal is
adapted and displayed in the spectrogram. It seems works, from now must
be implemented the fft.
Spectrogram is instantiated from mainwindow. Is this right?
Now a QwtRasterData raster data is used, a better solution could be use
QwtRasterMatrixData
Improvment in the fft
Avoid update frequency plot when some curve attribute change
Improved beahviour in  widget name
sample rate is calc changed from ceil to floor in calculation
fix a problem rwhen cancel is selected
fix an issue return a curve if cancel is selected
Fix an issue that the widget  wasn't update importing curves or opening project
Improved spectrogram

FFT implementation
Spectrogram option. Overlap is missing
fix a little minor error  in setting spectrogram axis
Changed design spectrogram data, now use QwtMatrixRasterData with bilinear resample
Basic Spectrogram implemented.

Trunk can be merged in master trunk
Merge pull request #3 from lawrence-iviani/spectrogram

Spectrogram