[Enhancement] UWP: Button: AccessKey support

[kingces95]

Rationale

Disabled and/or power users find the keyboard easier/faster to use than the mouse.

Implementation

A UAP platform specific subset of https://docs.microsoft.com/en-us/windows/uwp/design/input/access-keys as can be exposed via the following:

public class View {
	public static readonly BindableProperty AccessKey; // string
	public static readonly BindableProperty AccessKeyPlacement; // AccessKeyPlacement
}

public enum AccessKeyPlacement {
	Auto = 0,
	Top, 
	Bottom, 
	Right, 
	Left, 
	Center, 
}

Expected Result

https://docs.microsoft.com/en-us/windows/uwp/design/input/access-keys

Implications for CSS

None.

Backward Compatibility

None.

Difficulty : Easy

The test matrix should include all derivations of View typically clicked or tapped.

[henricm]

Should implementation be UWP only? If so, should the Bindable properties be added to a WindowsSpecific View class or shared View class?

[henricm]

I have started implemented this and I've got the basics to work.
So far I've put the bindable properties in the shared VisualElement class and updated the Windows VisualElementRenderer. I've tried it on Label and Button classes and it seems to work as expected. I have a few questions:

1. In which class do we want these properties - is VisualElement the right way to go?
2. Should we place them in a Windows specific VisualElement rather than the view class shared between platforms. I thought maybe it would be easier to use if we do it this way, instead of having to use the button.On<Windows>()... approach. But I'm not sure we want to add this to more platforms in the future?