CORE-3874 Fixed data drag-n-drop issues in v1.8.

The following changes allow items to be dragged into folders in the grid, allow folders to be dragged into one of their ancestor folders (except for their direct parent folder), and prevent items from being dragged when dragging the scroll-bar:
* DiskResourcePresenterImpl was passing src and target folders into DiskResourceUtil::isDescendantOfFolder in the wrong order.
* DiskResourcePresenterImpl::getDropTargetFolder was using the wrong Element in the view::findGridRow call.
* Added a DiskResourceViewDnDHandler::validateDropStatus method that needs to be called from the onDragMove method as well as onDragEnter (also called from onDrop for good measure).
* Added a DiskResourceView.Presenter::getDragSources method that verifies the drag event started from a valid item in the tree or grid before returning the appropriate selected items.
* Allow DiskResourceViewImpl::findGridRow to return null. This could prevent errant drops from moving items unexpectedly into a folder listed last in the grid.
* Prevent DiskResourceViewDnDHandler from calling the move DiskResources service if drag events go directly to onDrop without going through onDragMove first, to prevent rapid clicks from trigging a errant moves.