Warning message when closing the model editor

This is not ECA, this is the bpmn_io UI.

The close button is about to disappear from the UI as there is not really anything to close. What's happening is just that the user can go to a different page by clicking in the breadcrumb or in the menu.

This is the same as with any other form: when the user doesn't hit save but goes to a different page, the input to the form is lost as well.

This is hard to avoid, so I mark this as won't fix. But if anyone comes along with an idea on how to implement something like that, which works with any method of leaving the current page without prior saving, i.e. also for just closing the browser tab, is welcome to propose a technical solution for this. But then, this would probably even qualify for a module of its own, because that would solve many more issues than just for bpmn_io.