Provide better dirty state calculation for Markdown inlineEditors.
Getting the text contents of a CodeMirror widget every key press is

expensive, and resulted in some noticeable typing lag. Now that

inlineEditor allows callers to override dirty state calculation,

MarkdownEditor can provide its own implementation that just asks

CodeMirror if it's dirty. This removes the typing lag.

Tested that the new handler was being called and the old one was not.
Typed a bunch and saw that the lag was gone.