Does somebody happen to know, if there is a way to add http://draw.io drawings into a Plone5 site, so that the diagramm is displayed as Image, the editor opens on double click and writes back the changed diagramm.
Is there an AddOn that does something similiar with another content type?
draw has an embed mode. Using this you can open draw.io in an iFrame overlay and have comms between the host and draw to load/save, get PNG, etc. This is the simplest way to get an integration going.
If you want draw running locally, that's a little more complex. You could take the auto-built .war of drawio and running it on a Tomcat/other servlet engine. Then you'd reference the URL of that engine rather than the online draw site.