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. 
So I this was my initial plan as it seemed less janky - but I found the reference to be inconsistent between the various states Zotero could occupy. For example, the `window.opener` reference when Zotero occupies the lower portion of an existing page (as opposed to an individual tab) is to `chrome://browser/content/browser.xul`. To keep things consistent between these different states, I explicitly passed it in. I could be missing something here, however…