RadioButtons: Wrap custom ItemTemplate in a RadioButton if required

[Felix-Dev]

Description

This PR fixes an issue where a custom ItemTemplate specified for the RadioButtons control would not wrap that template in a RadioButton if it wasn't a RadioButton. This made the RadioButtons control basically unusuable if developers did not explicitly specify a RadioButton as the root of the ItemTemplate (no (mutually-exclusive) selection was possible).

We now wrap the ItemTemplate in a RadioButton if it isn't already a RadioButton. If the user set a RadioButton as the root of the ItemTemplate we take it as is and perform no wrapping. This will make sure user-provided RadioButton customizations won't be lost.

@kikisaints As part of #3159 providing better documention for the RadioButton customization aspect was identified as a work item. My curent thinking is to create a new sub-section titled "Customizing the radio button appearance" here. This section will show you can explicitly set a RadioButton as the root of the ItemTemplate to change its appearance:

public ObservableCollection<string> Options { get; set; } = new ObservableCollection<string>() { "Option 1", "Option 2" };

<muxc:RadioButtons ItemsSource="{x:Bind Options}" SelectedIndex="0">
    <muxc:RadioButtons.ItemTemplate>
        <DataTemplate>
            <RadioButton Foreground="Blue">
                <TextBlock Text="{Binding}"/>
            </RadioButton>
        </DataTemplate>
    </muxc:RadioButtons.ItemTemplate>
</muxc:RadioButtons>

I would also like to create a reference on the RadioButtons API documentation page to the RadioButton control overview page as can be seen for other controls like the TabView (see the green info block).

I would work on this once this PR has been merged and would tag you then so you can give further input/review. For now, you can already comment on how you view the overview I gave above, if you like.

Also @ranjeshj FYI.

Motivation and Context

Fixes #3159.

How Has This Been Tested?

Tested visually and added API test.

Screenshots:

The following C# and XAML markup gives us the UI shown below:

public ObservableCollection<string> Options { get; set; } = new ObservableCollection<string>() { "Option 1", "Option 2" };

<muxc:RadioButtons ItemsSource="{x:Bind Options}" SelectedIndex="0">
    <muxc:RadioButtons.ItemTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding}"/>
        </DataTemplate>
    </muxc:RadioButtons.ItemTemplate>
</muxc:RadioButtons>

[StephenLPeters]

/azp run

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[StephenLPeters]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]

@Felix-Dev The pipeline fix just went in, could you please merge master into this PR?

[Felix-Dev]

@StephenLPeters Merged with master.

[StephenLPeters]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[StephenLPeters]