Retrieve Metadata query limit fixes

It doesn't seem that we were recognizing HTTP 503 responses from GS as Query Limit Reached.

Also added captcha support. Works surprisingly well on my tests.

Some other tweaks:
* Don't stop retrieving metadata if we hit a limit (with this patch, only if the user fails/refuses to solve captcha or GS captcha mechanism changes and we don't recognize it), because we may still be able to retrieve ISBN-/DOI-based metadata
* Don't close window on blur. I'm not sure why this was ever a good idea. Sometimes you may want to analyze the retrieved metadata and see if everything is right. Since the dialog box is not modal, you can actually scroll through your items in Zotero, which is nice. IMO, there's really no good reason to close the dialog automatically.

I'm sure this will need some code restructuring, but I think the general idea works extremely well.