Support drop to start/end of tree and to parent of branch

[joelpt]

• Support dropping at end of tree by allowing drop onto parent/body of .ftRoot; this should always add the dropped nodes as the last item under the tree root. Currently we can only drop the nodes 'after' the last node in the tree at the last node's branch depth; and the nodes must be dropped onto the last node, not into the empty space.
• Support dropping at the start of the tree by allowing drop onto parent/body of .ftRoot.
• Support dropping 'after' the currently hovered node's parent by dropping either over a node's .ftExpander or just to the left of its .ftItemRowContent element.