Marked skipped lines with a gray line

Also avoid naming conflict (DocNode.copy)
It's possible to add comments now

It's not possible to see, edit or otherwise do anything with them :-(
Fix hover position
Comments are now shown as well
Html is not escaped in comments

I changed my mind: we don't need to be paranoid here, let the user do
whatever he wants to do. So why not let him use html code in comments?
It's possible to delete comments