[feat] Collaborative comments

[miqmago]

First of all thanks for this really great application!!

Right now the collaboration comments are only available on the online app.diagrams.net when shared through google drive. I could not find live comments on nextcloud integration. In the online version there is only one conversation dialog for all the application.

It would be really awesome if we could add comments anywhere in the drawing as conversation objects that could be moved anywhere and would show up compact until you click on them to expand and see the entire conversation.

I think it would be useful to keep conversations in a sandbox, so that each conversation could be independent from the others, and any user could add a new conversation/comments box. In this way the conversations would remain organized separately.