Feature/media queries

[danwalmsley]

What does the pull request do?

Prototype for implementation of css like media queries.

Essential to support multiple form factors.

What is the current behavior?

What is the updated/expected behavior with this PR?

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

Checklist

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

Breaking changes

Obsoletions / Deprecations

Fixed issues