Improve inset plot in H(f) widget

[chipmuenk]

The optional inset plot in the H(f) widget has some quirks:

• When resizing the window, warnings are generated "This figure includes Axes that are not compatible with tight_layout, so its results might be incorrect." [from matplotlib/figure.py:1742]
• Inset plot should have useful presets, depending on filter type. "Stop Band" or "Pass Band" could be useful presets, lin / log scale should be selectable
• Position and size of inset plot should be selectable, e.g. with numbers similar to the legend position mechanism in matplotlib
• Sometimes using the Home button can crash the app, probably after creating the inset ans / or using lock zoom in matplotlib/backend_bases.py line 3122 in _update_view() : a.set_view(views[i]) with
  IndexError: list index out of range