Improve extensibility

[SuperJMN]

I wanted to create a feature for TreeDataGrid and found that it was almost impossible to do it without having at least:

• A way to get the items in a Source
• A way to get the children in a Source (if any).

Without those, people like me would have to force the type system beyond reasonable in some scenarios.

Take this as an example:
https://github.com/SuperJMN-Zafiro/Zafiro.Avalonia/blob/master/Zafiro.Avalonia/ScrollToSelectedItemBehavior.cs

As you see, this behavior is not straightforward to use because:

1. It needs a type parameter (T).
2. It needs to know how to get the children for T, hence the constructor argument that uses reflection on T to get the given property.

Sample usage would be:

<TreeDataGrid Source="{Binding Source}">
    <i:Interaction.Behaviors>
        <global:ScrollToSelectedItemBehavior x:TypeArguments="vm:Model">
        <x:Arguments>
            <x:String>Children</x:String>
        </x:Arguments>
        </global:ScrollToSelectedItemBehavior>
    </i:Interaction.Behaviors>
</TreeDataGrid>

T being:

class Model 
{ 
    ...
    IEnumerable<Model> Children { get; } 
    ...
}

With the changes I'm suggesting, the Behavior would stop requiring type parameters and reflection to know how to get the children.

IMHO, this is beneficial from an extensibility standpoint.

I hope you accept these changes :)

[dnfadmin]

All CLA requirements met.

[grokys]

Couple of nits you may want to fix, but LGTM!

[SuperJMN]

Nits included :)