Add RadioMenuFlyoutSubItemStyle

[teaP]

This change does two main things:

Adds the RadioMenuFlyoutSubItemStyle, which -- if applied to a MenuFlyoutSubItem -- will show a radio dot if a RadioMenuFlyoutItem in the menu is checked. It does this by setting the new attached property AreCheckStatesEnabled, which runs a check when the item is Loaded and sets the appropriate visual state.

Also, the change in code to RadioMenuFlyoutItem makes it so that items don't have to be together in a menu in order to be grouped together. Instead of going through the visual tree to find siblings, it just keeps track of which item is currently checked, and unchecks it when a new item gets checked.

[teaP]

/azp run

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[teaP]

/azp run

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[teaP]

/azp run

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[jevansaks]

New APIs need to be added in [MUX_PUBLIC_V2] block

[teaP]

ok, done

[jevansaks]

Not quite -- this makes the attribute apply only to the next member. You need to put it in { } around the thing:

        [MUX_PUBLIC_V2]
        {
            [MUX_DEFAULT_VALUE("winrt::ControlsResourcesVersion::Version1")]
            ControlsResourcesVersion ControlsResourcesVersion{ get; set; };

            static Windows.UI.Xaml.DependencyProperty ControlsResourcesVersionProperty{ get; };
        }

[ranjeshj]

@jevansaks I've updated this. We need to add the metadata test to catch these type of issues.

[ranjeshj]

/azp run

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[ranjeshj]

/azp run

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[ranjeshj]

/azp run

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