Editor: URL dialog broken

[bill]

After refactoring of DropDownButton?, the URL TooltipDialog? isn't working anymore. Needed to be redesigned anyway.

I think we need to start with a custom widget (which extends TooltipDialog?) and contains the dialog for adding a URL, with attach points for save and cancel button.

Then the button can reference that dialog widget, something like

new DropDownButton({label: "hi", iconClass: "foo", popup: new UrlTooltipDialog});

[bill]

This is mostly fixed (see recent checkins marked with #2820), but a few more things to do:

• populate url according to selection (if cursor is over a url)
• disable button when nothing selected

[liucougar]

(In [9878]) fixes #3635 and #3889