Cleanup dropdown lists to use new DropdownEventHandler class

[redmunds]

During development of CSS Quick Edit New Rule user story, I added a DropdownEventHandler class (utils/DropdownEventHandler) as a place to put common functionality for dropdown elements. But all of the other lists I looked at had just enough difference that it wasn’t easy to migrate:

• Code Hints – Enter key handling when there’s “pending text”
• Recent Projects – button to “remove from list”
• Context Menus – currently have no keyboard support

So, we should take another look at consolidating some of this code.

[njx]

Low priority code cleanup to @redmunds. Might also want to think about whether this is related to the general popup manager cleanup work in (#1381) (which was moved to backlog) or whether it's orthogonal.