Marshal Color Type between Swift and .NET

[CartBlanche]

• [x] Add .background glue code and associated extension code.
• [x] Use Colour type to marshal View's Background View
• [ ] Add ViewModifier Conformance protocols
• [ ] See if we can modify the main view, via the ViewModifier

[CartBlanche]

Rebasing has messed up things. I'm going close this PR and CP the commits over to a new branch, rather than try to fix things here.