StretchDirection for Viewbox

[MarchingCube]

What does the pull request do?

Implements StretchDirection as in WPF/WinUI https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.viewbox.stretchdirectionproperty?view=net-5.0

What is the current behavior?

Viewbox content is always scaled.

What is the updated/expected behavior with this PR?

One can control stretching behavior.

How was the solution implemented (if it's not obvious)?

Current Viewbox.Stretch is a bit problematic since it is a direct property for some reason. For backport reasons I won't change it in the current PR but I will create a follow up one with a breaking change.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Avaloniaui.net with user documentation

[grokys]

LGTM, I was just about to implement this myself.

Love the new interactive demo in ControlCatalog. I think we can remove the old contents from that page.

[MarchingCube]

When backporting - please include #5918.