Blender Git Loki

Cleanup: Simplify node editor link dragging storage

Now that `node_intern.hh` is a C++ header, we can use C++ types
there. This patch replaces the linked list of dragged links with a
vector. Also, the list of drag operator custom data, `nldrag`, doesn't
seem to need to be a list at all, so I just made it a unique pointer.

Differential Revision: https://developer.blender.org/D13252