Update TreeView to support blocking drag&drop for specific items

[kaiguo]

Description

Enable TreeView to block drag&drop on certain items.

Added checks for "handled" in drag&drop callbacks so developers can set "handled" to true to skip TreeView built-in drag drop logic.

Also added a "NewParent" property in TreeViewDragItemsCompletedEventArgs, which makes it easier to figure out the drop position.

Motivation and Context

Fix #381
Fix #1154

How Has This Been Tested?

Updated existing test to include "NewParent" property.

[StephenLPeters]

You should try to avoid scratch variables if possible, I might suggest the following:

const auto newParent = [items = args.Items(), listControl = ListControl(), rootNode = m_rootNode.get()]()
{
    if (listControl && items && items.Size() > 0)
    {
        if (const auto draggedNode = listControl->NodeFromItem(items.GetAt(0)))
        {
            const auto parentNode = draggedNode.Parent();
            if (parentNode && parentNode != rootNode)
            {
                return listControl->ItemFromNode(parentNode);
            }
        }
    }
    return static_cast<winrt::IInspectable>(nullptr);
}();

[kmahone]

[kaiguo]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[AignerGames]

Might be a dumb question, but I want to enable the general reorder logic but need some specific checks:
A: prevent drag/reorder child nodes if they are under a specific parent
B: don't allow node drops which are dragged from other parent nodes (which allow drag&drop) under this specific parent

It sounds like this change made it possible to do so, but I don't understand how I would implement it with this change, can some one give me a example or some hints?

[StephenLPeters]

@AignerGames I think that the answer is to set the CanDrag and AllowDrop properties on that parent node to false.