#37 Changing window kind does not update spectrum

[Giampiero Salvi]

Hi,
in the spectrum section, whenever a parameter is changed, the spectrum is recomputed and updated in the plot. However, this does not work with the "Window" parameter. For example, changing from Hamming (default) to Rectangle has no effect, unless the user clicks again on the original sound pane.

Sometimes it is useful to show the effect of the windowing on the spectrum, and it is kind of tricky to click on the exact same time instant in order to compare the two spectra.

Tested with WaveSurfer 1.8.8p4 on Ubuntu 16.04.