Add functionality to permit "dragging" tree nodes on top of one another in order to insert a child node

[jleidel]

This will insert a child node dependency. Ideally, we would query the source (item being dragged) and the destination (which will subsequently become the parent) in order to determine if the target nodes have special insertion functions (eg, InsertRegister(...)

[jleidel]

This should register and handle the EVT_TREE_BEGIN_DRAG event; see also wxTreeEvent::Allow()