Losing data when a user click the back button in the browser

[Sander3003]

Is your feature request related to a problem? Please describe.
As an engineer
I want to be informed to save the data first before I click the "back" button in modern browers
So I do not lose my changes

Describe the solution you'd like
Show a warning when a user want to back in the browser.

Requirements
Only show the warning if a document is open in OpenSCD

Suggestions for a specific user interaction
A pop-up window should show up:
Are you sure? Your work will be lost

Additional information
Example:
https://stackoverflow.com/questions/38306510/alert-on-browser-back-button-click-or-page-refresh