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A quick sanity check on possible UI features is to search available Qt (GDC) and HTML (GWC) documentation and see if what you propose is possible.
So for something like this, these links ...
http://doc.qt.io/qt-5/qcombobox.html#insertSeparatorand
http://jsfiddle.net/JFDgH/2/ ... suggests that the concept of a ComboBox separator is possible for both GDC/GWC. There is more than it to that (AUI Tree, FGL syntax changes - ui.ComboBox.addSeparator()?, QA, documentation etc) but doing something like that gives you a rough idea if something is possible and may even allow you to implement in GWC via snippet changes
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