Silence react@15.5.0 warnings

Add prevTreeIndex, nextTreeIndex to canDrop callback

Fix expand button behavior in forms

Add canDrag API, fix horizontal scrolling

Add canDrop API, fix react@16 warning

Fix hover errors. (Thanks to TooHTooH) Fixes frontend-collective#40

Provide SortableTreeWithoutDndContext variable to avoid react-dnd issue

Handle case when rowHeight prop value is a callback

Add option to render without virtualization

Fix visual bug with react-virtualized >= 8.6.0