edit extra field of parent item with lastModifiedTime

I want to be able to track whether I’ve read an item by sorting on the
date.  Since Zotero doesn’t allow editing the Date Modified time, the
next best thing is to just edit the parent record, so the date modified
time changes to the current time.  This is a hacky solution, and we
lose information, but we also gain the knowledge that we’ve read the
paper.