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. 
If this is just testing ```link```, it can go above.

But I'm not sure that just hitting Return should close the window, and typing a space and hitting Return definitely shouldn't.

I think if cleanAttachmentURI the URI is empty (or trimmed and empty), it's just an invalid URI, and the dialog should stay open, the contents of the field should be selected, and ```pane.items.attach.link.uri.unrecognized``` should be displayed.