Try factoring out the dropdown boilerplate

[kythyria]

Related to #2587, this is a possible way to refactor the UI for dropdowns and radio buttons based on built-in enums.

There's this much indirection for a few reasons:

1. To account for the fact that BlendMode has two different lists associated with it
2. Using instance rather than associated methods might help with user-defined enums
3. It'll reduce the refactoring required of built-in enums when that happens.

Probably need to move the definitions of AsU32 and DropdownableSourceStatic, that's kind of a meh name if nothing else.