Start making everything nullable so we can unset and use defaults

[mattleibow]

Description of Change

Implements #

Additions made

• Adds

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might effect accessibility?

• APIs that modify focusability?
• APIs that modify any text property on a control?
• Does this PR modify view nesting or view arragement in anyway?
• Is there the smallest possibility that your PR will change accessibility?
• I'm not sure, please help me

If any of the above checkboxes apply to your PR then the PR will need to provide testing to demonstrate that accessibility still works.

[charlesroddie]

This is a large increase in complexity. And it prejudices Maui towards the Xamarin approach where things looking different per-platform is the default.

It's not needed because whatever this "default" is, someone who wants this "default" value can get it before creating the Maui object.

[PureWeen]

Decided to go with the exact opposite approach for now :-)
#1485