gui comp scalewindow: force minimum size to ensure everything is displayed

apparently with wx3.0, the default minimum size is smaller, so we really need to force it.

also do various clean ups
gui comp combo: remove workaround not necessary anymore

with wx3.0, left/right work as expected.
gui: small changes to sizes

partly due to update to wx3.0, some default sizes changed and so some sizes
needed some tweaking. The legend is now slightly less high, on purpose.
gui: get (almost) rid of the workaround combobox.GetSelection()

with wx3.0, cb.GetSelection() works almost as expected.
The only problem is that if the combobox has free text, GetSelection() will
return the last selection, and not wx.NOT_FOUND as expected.