Add ZU.dom2text and Zotero.Utilities.Internal.getDOMDocument
* getDomDocument: returns a detached DOMDocument object
* dom2text (TODO): Currently just returns Node.textContent, but is intended to return Zotero-formatted string based on text formatting in the DOM and the Zotero.Item field that the text is meant for
Don't count on 0 being the user libraryID in Zotero.Libraries
Some other minor tweaks
Virtualize data objects