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. 
I'm ok with centering it on initial display, but not afterwards. It's possible that a user moves the dialog in such a way that centering it positions the Cancel button right under the mouse cursor, so double-clicking (possibly accidentally) OK would end up cancelling the dialog (had that happen to me a few times...)