ComboBox megawidget with adjusted width

bll 2017-9-20: This widget will automatically adjust the width of the combobox to match the values supplied. Good for localized test.

It has an internal cache, as in my particular use case, I have certain screens that display the same combobox multiple times. This can be turned off or removed.

The -popdownfont argument is an addition and can be used to set the popdown listbox's font. The code is set up to do this once the popdown listbox is mapped. (If the popdown listbox is force-created early, it may display in the entirely wrong place on the screen).

bll 2018-10-1: 1.3: Remove tailcall, does not work.

bll 2018-9-6: 1.2: Change to use tailcall.

bll 2018-3-16: 1.1: Make the bindtag more specific.