Show modified save icon if URL/DOI exists in library

Warning somehow when people are about to save an existing item is a common request (most recently here). It's problematic in general, since data isn't available without running the translation process, which we can't really do during normal browsing (too slow, too many site requests, etc.). But there's some low-hanging fruit here that we should probably do — specifically, checking for an existing URL or DOI in the library.

One option here would be to have detectWeb() return an object that included one or more properties with arrays of ids, in addition to itemType. But we'd have to add support for that in advance of rolling out any translators that used it, or else we'd break detection for older versions. Another option would be to have detectWeb() call a certain function with ids it found.

The connector would then have to check the target library for those ids via a call to Zotero or the API.