Add a WinUI3 backend

[jhi2]

(Ticket content has been edited significantly from the original to provide better guidance on the feature request)

What is the problem or limitation you are having?

Toga's look and feel on Windows currently uses Winforms, which doesn't reflect the current look and feel of Windows apps.

Describe the solution you'd like

We should add a WinUI3 backend.

Describe alternatives you've considered

Continue to live with Winforms.

Additional context

The prerequisite for this backend is the development of Python bindings for WinUI3.

[freakboy3742]

Thanks for the suggestion - we'd love to add a WinUI3 backend; the problem is that there are no Python bindings for WinUI3. Once those bindings exist and are stable, a WinUI3 backend for Toga can be developed.

[jhi2]

Maybe I could help make one