ComboBox megawidget - return associated values

bll 2017-9-20 This combobox displays one set of values and returns the associated value from another list. e.g. If you have a list: January, February, ..., December, you can have it return 1-12. Good for localized text.

The -returnvalues argument is required and specifies the list of values returned by the combobox. The get and set sub-commands will return and should set a value from the -returnvalues list.

If you re-configure -values, be sure to reconfigure -returnvalues at the same time.