Support lazy child nodes loading in tree view

[harry-xi]

In some of case, we will display a file tree from web or another remote place, which mean we need time to fetch the hall tree. support lazy load of child nodes will let user have a fast initial load, and may also save memory when some nodes have a huge number of child nodes.

In current design of the TreeView and ITreeItemsView, it is possible to build this on user own, but it is a complex task and need do some work in ItemTemplate and the backend ITreeItemsView at same time, using node to transfer the loading state to display the loading state. This is the reason why I think we need a control for this.

[al6uiz]

Please refer to #167.