Support headline backlinks

Hi, I like some of the trendy ZK ideas but I'm not 100% sold. For example I don't think it's worth losing many features org mode offers for single file (or at least not thousands of files) setups. Also I dislike metadata overhead and I believe that with some care linking to headlines is just fine, I don't want to have property drawers everywhere, wikis has been linking to filenames for decades, moreover having backlinks at sight makes it easy to rename them in case it's necessary.

I was glad to learn that this project implements a backlink facility without requiring or promoting a many-files workflow, but it was a bummer to find out that it still requires id links. I don't know how difficult it's to add headline backlinks to the sidebar, but it would be nice to support different workflows.