Creating itemsControl child elements with the ability to resize using a splitter

I want to insert widgets into my own ItemsControland make them resizable. How do I achieve this?

This is my XAML:

<ItemsControl ItemsSource="{Binding TestForList, Mode=OneWay}">

    <ItemsControl.ItemsPanel>
        <ItemsPanelTemplate>
            <StackPanel Orientation="Horizontal"                
                        VerticalAlignment="Stretch"                
                        HorizontalAlignment="Stretch" />
        </ItemsPanelTemplate>
    </ItemsControl.ItemsPanel>

    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <Border Margin="5" 
                    BorderThickness="1" 
                    BorderBrush="Black">
                <TextBlock FontSize="100" Text="{Binding}" />    
            </Border>                    
        </DataTemplate>
    </ItemsControl.ItemTemplate>

</ItemsControl>

What is associated with:

public List<string> TestForList
{
    get
    {
        return new List<string> { "A", "B", "C" };
    }
}

I want to somehow add separators between the elements so that they can be changed. Is there something built in to achieve this?