Pass current treeData to canDrop callback

[florianf]

Hi,

currently the prevTreeIndex, nextTreeIndex indices in the canDrop callback are wrong when nodes are expanded during the dragging operation (hovering over closed node). The changes are internally reflected in the treeData structure, but not yet published (via onChange or onVisibilityToggle). This breaks all invocations of getVisibleNodeInfoAtIndex when using f.ex. nextTreeIndex.

To mitigate the issue, the current treeData structure is passed to the canDrop callback.

Thanks for your work!

[coveralls]

Coverage remained the same at 76.492% when pulling 1dfbab1 on florianf:master into d55561e on frontend-collective:master.