Various issues with FFT window widget

[chipmuenk]

• When the number of data points is changed in the filter order / the number of data points in the impz widget, this should be reflected in the win_fft widget (currently only works when the window type is edited)
• When the filter order is changed in windowed FIR design, this should be reflected in the fir_fft widget. Currently, it is only updated when the window type is changed.
• The windowing function cannot be modified for the spectrogram. The FFT-Window widget should be rewritten to allow a variable number of data points, it should also be possible to modify not only view the FFT window parameters or centered at N = 0
• include second combo box in FFT UI e.g. for cosine windows with varying number of terms and to integrate Bartlett and Triangular window in one entry