Add PopupButton Widget

[ecton]

This issue is tracking support for a widget that displays a selected value. When clicked, a menu of options is shown. If a new option is picked, it is shown in the button.

Other frameworks sometimes call this a DropDown button.

[hydra]

DropDown widgets, aka 'Select' in HTML are highly useful, however if time is a factor an implementation of the Amiga's 'cycle gadget' would also be welcomed in lieu or in addition to a DropDown / PopupButton.

https://wiki.amigaos.net/wiki/UI_Style_Guide_Gadgets#Cycle_Gadgets

They look like this:

When you click them, they just cycle to the next item in the list. Like radio groups, they work well for shorter lists, but not for longer lists and are space-saving when compared to a radio group.

They render in-place and there are fewer edge-cases than with the DownDown/PopUpButtons. One common edge case with DownDowns is where/how to render the menu if any part of the pop-up menu that appears when you press the button doesn't fit in the application window.