Allow drag-n-drop (including between trees)

[grapoza]

https://html.spec.whatwg.org/#dnd
https://developer.mozilla.org/en-US/docs/Web/API/HTML_Drag_and_Drop_API

ARIA drag-and-drop attributes are deprecated in ARIA 1.1 and will not be implemented.
https://www.w3.org/TR/wai-aria/#attrs_dragdrop
https://www.w3.org/wiki/PF/ARIA/BestPractices/DragDrop
https://dev.opera.com/articles/accessible-drag-and-drop/

[grapoza]

This should include emitting treeViewNodeDrop when a treeview node's data is dropped into the tree

Multiple selected nodes at once? If so, how does it handle nodes selected across different subnodes?

Add extra drop targets (1/10th of node height?) to nodes to allow adding children into collapsed/childless nodes?