Make sure we can reopen secondary windows

[tsmaeder]

What it does

There were two problems:

1. A listener for the 'close' even on secondary windows went missing in the browser case when introducing secondary window support for Electron
2. The code uses a listener for the 'close' event on the secondary window. However, this event does not exist. So basically, this never worked 🤦

Fixes #13214

Contributred on behalf of STMicroelectronics.

How to test

Open and close secondary windows in both the browser and electron case. Make sure there are no exceptions and documents can be reopened. I used the hediet.vscode-drawio extension from open-vsx to test.

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[rschnekenbu]

Looking good to me!
I tested with the same extension, on electron and browser. No issue to open in new window /close/reopen a drawio editor on both versions.