Provide selected item for a ComboBox

[wolfram]

This simple patch provides the selected item for the combobox widget, as discussed with bill on IRC. (Passing the item as second paramter to onChange() would be too complicated, since that is done in one of the base classes :-( I didn't find an easy way?)

You can now do, after one item had been selected:

    var item = widget.item;

in order to retreive the data from the item you can now (or have to) do the following

    var name = widget.store.getValue(item, "name");
    var someValue = widget.store.getValue(item, "someAttribute");

[wolfram]

provide the selected item

[bill]

(In [10668]) Expose currently selected item as ComboBox?.item. Patch from Wolfram (CLA on file). Fixes #4596.

[bill]

(In [10669]) Expose currently selected item as ComboBox?.item. Fixes #4596.