#6 Allow drag and drop to the bottom of the list

[Will at Mavenomics]

This patch allows dragging and dropping of nodes to the end of the treeview. If the list is empty, dropping onto the treeview targets the root node. If the list isn't empty, dropping onto the area past the last node is treated the same as dropping past the last child of the root node.

This patch has two parts: a change to the sample app to demonstrate the first (empty) case and a change to TreeViewAdv.SetDropPosition() to allow the non-empty case.

[Will at Mavenomics]

Patch to allow drag and drop to the end of the list