Add a WinUI3 backend

(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 use Winforms.

### Additional context

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

Start by reviewing Toga's existing Winforms backend and the prerequisite work for Python bindings to WinUI3. The feature is done when Toga has a functioning WinUI3 backend that provides a more current Windows look and feel.

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