Support for overriding Content attribute in derived classes

[yevgeni-zolotko]

Hi, thank you for this library. Consider the following situation:

    class Base
    {
        [Content]
        public Base[] BaseProp { get; }
    }

    class Derived : Base
    {
        [Content]
        public Base[] DerivedProp { get; }
    }

With this code I would like to override the "content" property in Derived class to use DerivedProp instead of BaseProp. Then when parsing the following XAML:

    <Derived>
        <Base />
        <Base />
    </Derived>

I get error: XamlX.XamlTypeSystemException: 'Content (or substitute) attribute is declared on multiple properties of Derived'

Commenting the following code here:

                if (GetCustomAttribute(p, TypeMappings.ContentAttributes).Any())
                {
                    //if (found != null && !p.Equals(found))
                    //    throw new XamlTypeSystemException(
                    //        "Content (or substitute) attribute is declared on multiple properties of " + type.GetFqn());
                    found = p;
                }

seems to solve this problem for me.

Maybe there is a different better way to achieve this? If not, should we cover this case in the library? I can submit a PR if needed.