[Enhancement] Add Visual progress behind feature flag

[PureWeen]

Description of Change

Add a Visual property to VisualElement that allows us to swap out different renderers based on a Visual Type property

fixes #4435

This PR also adds a starting set of Material Renderers as the first use case for this behavior

• Button
• Entry
• Frame (CardView)
• ProgressBar (iOS)

API Changes

Added:

public class VisualElement
{
		public IVisual Visual
		{
			get { return (IVisual)GetValue(VisualProperty); }
			set { SetValue(VisualProperty, value); }
		}
}

public sealed class ExportRendererAttribute : HandlerAttribute
{
		public ExportRendererAttribute(Type handler, Type target) : this(handler, target, null)
		{
		}
}
// Example Usage
[assembly: ExportRenderer(typeof(Xamarin.Forms.Button), typeof(Xamarin.Forms.Platform.iOS.Material.MaterialButtonRenderer), new[] { typeof(VisualRendererMarker.Material) })]
namespace Xamarin.Forms.Platform.iOS.Material
{
	public class MaterialButtonRenderer : ViewRenderer<Button, MButton>

Platforms Affected

• Core/XAML (all platforms)
• iOS
• Android
• UWP

Before/After Screenshots

There should be no visual changes to standard Default Renderers
Here is the view when using Material Visuals

Testing Procedure

• Make sure Visuals are hidden behind the experimental flag correctly
• Make sure all current renderer behavior has been maintained (lots of changes to registrar) so that this change isn't breaking

PR Checklist

• Has automated tests
• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard

[StephaneDelcroix]

Except for the part where you shuffle stuffs that were not misplaced, I think that the Core stuffs are nice.

[samhouts]

No blockers!

[StephaneDelcroix]

no blocker.

[StephaneDelcroix]

we don't usually override OnPropertyChanged but hook into the BP propertyChanged:, but this is fine