Add the ability to modify document title collaboratively

For an upcoming project, I'm embedding the Editor into a larger UI that has an editable "title" field. I'd like users to be able to collaboratively change the title.

Also of use is "last edited by" information.

For that, the Editor needs to provide an event that lets the embeddor know whenever metadata changes, and a method to set the title of a document.

This PR does that.