Remove references to an "app window"

[mattleibow]

Description of Change

Remove the "app window" references on app objects. Not only is this a bit confusing in multi-window scenarios, we can now make use of the Application.Windows[x].Handler.NativeView to get the actual window.

Anything not in that list of windows has no native window nor handler yet and thus would not be found anyway.

A benefit of not looking on the Maui* objects for properties is that any activity/window type can be used (maybe from a 3rd party) and it is all linked via the handler.

Additions made

• Removed MainWindow and VirtualWindow properties

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 affect accessibility?

• Does this PR introduce a new control? (If yes, add an example using SemanticProperties to the SemanticsPage)
• APIs that modify focusability?
• APIs that modify any text property on a control?
• Does this PR modify view nesting or view arrangement 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.