Change the font used by checkboxes to match UWP

[Difegue]

GitHub Issue (If applicable): #22

PR Type

• Bugfix
• Style fix

What is the current behavior?

Checkboxes in Uno.Playground use the WinJS Symbols font, leading to the wrong styling.

What is the new behavior?

The checkmark uses Segoe MDL2 Assets instead.
I'm pretty sure this is an absolutely horrible way to fix this, and that changing the default font used in Uno.UI for the Checkbox control would be better.

...Thing is, I couldn't figure out how to fix it there 😅, or if that's wanted. (MDL2 being closed source and all.) Guidance would be most welcome!

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] Tested code with current supported SDKs
• [ ] Docs have been added/updated which fit documentation template. (for bug fixes / features)
• [ ] Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• [x] Wasm UI Tests are not showing unexpected any differences. Validate PR Screenshots Compare Test Run results.
• [x] Contains NO breaking changes
• [ ] Updated the Release Notes
• [x] Associated with an issue (GitHub or internal)

Other information

Internal Issue (If applicable):