Hello Rostislav, Nice Module. Thanks for the work.
I went through your module...

Hello Rostislav, Nice Module. Thanks for the work.

I went through your module. It was nicely written.
No problems were found with the coding standards.

Just one usability issue.
Whenever a node(or any entity) is edited and editor changes( by changes i mean adding/editing/deleting) a note and do not save the node.
Since you are making the changes(add/edit/delete) in editor_note table prior to saving the node, the changes do not require
the node to be saved before taking effect. This seems to be an issue to me.
I think you should save the changes in the $form/$form_state and when the node is submitted, you can persist those changes in the editor_note.

Apart from that i think this module is very helpful. Great Job.