Make HTML source available on HTML editor

For now, there are a lot of issues/bugs with HTML Editor that prevent users from properly creating HTML content.

A quick workaround that would be acceptable while bugs aren't resolved would be the ability to switch to the HTML source.
This would allow users to directly add or edit the HTML.

UI
If possible, add an action to the HTML editor's menu bar to see HTML source.
Else, add an toogle ation next to save to switch to source when the format is HTML