Provide the CloseWindow warning experience for the 'X' button

[KaiyuWang16]

This is a follow up project of #1589 , which warns users of multiple tabs opened when press ALT+F4.
Now we are going to connect this experience with the 'X' button of the window.

For now, this PR only includes the implementation for IslandWindow (client window). Non-client window experience is still under implementation.

References

PR Checklist

• Closes #xxx
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Detailed Description of the Pull Request / Additional comments

To Test:

1. Start terminal.
2. Open multiple tabs.
3. Click 'X' on the top right corner of the window.
4. You should be able to see a warning. If you cancel it, then the Terminal keeps running.
5. If you click 'Close', then the app will close.
6. If you only have one tab opened, and click 'X', the app should close directly without any warnings.

[miniksa]

Looks good to me. I'm fine with this.

[DHowett-MSFT]

please make sure your pull request names are reasonable ("dev/kawa/1589 do the thing" is not a great branch name)

Pretend that they are completing the sentence, "This pull request will ..."

[KaiyuWang16]

please make sure your pull request names are reasonable ("dev/kawa/1589 do the thing" is not a great branch name)
Pretend that they are completing the sentence, "This pull request will ..."

PR name changed. I wonder if I need to change the branch name ? This may require to post a new PR.

[DHowett-MSFT]

No, you can keep the branch name.

[DHowett-MSFT]

Thanks!

[ghost]

🎉Windows Terminal Preview v0.6.2951.0 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Store Download