PDTree: Drag and Drop

[davidnmbond]

It should be possible to drag-and-drop nodes to nodes (to move a node to a child node of another node) OR the gaps between nodes and that this should trigger an event.

This should not perform any action, simply it should trigger the event.

[MarkGoldring]

Have added extra parameter: AllowDropInbetween that when set to true sets a 'before' argument in the Drop event to either true (before), false (after) or null (on). This extra level of information can be used to implement a drag and drop re-ordering. This can be seen in the new Tree 3 demo page.