Adding a sidebar (or bottombar) to the message compose window

[ndvbd]

Hi! I saw the example of "Compose Body" that adds a button to the top menu. However, is there an example showing how to add a sidebar (or bottom bar) to the text area (like in Word, Outlook or Google Docs)? I am talking about a sidebar that one can build using html+js that can use the thunderbird api to manipulate the text area (email body) content.