Link URI re: issue 486

The proposed changes address the silent failing of adding a link to an unrecognized URI. They allow the user to correct any potential mistakes made to the initial address through a prompt dialogue. 
Interesting (and I've wanted a way to get .dtd strings in JS for years), but that seems too hacky to me. Let's just get the value from the parent window DOM by id, which is probably less likely to break.