Citation dialog: Suggest page number for open documents

https://forums.zotero.org/discussion/132711/feature-request-suggest-current-page-number-in-citation-popup

Need to think about how to present this. I don't think we'd want to just auto-populate a page number for any open tab, but we should make it easy to select the current page via both keyboard and mouse. Need to do it in some way that doesn't conflict with the placeholder we're now showing ("Type 10–15…").

Not sure what to do about non–page numbers. @mrtcode, do we have any automatic detection for other locator types?

(Also, the current tab shows up in Selected Items rather than Open Documents, which is a bug. I'll fix.)