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

source/blender/editors/space_node/space_node.cc

@@ -303,7 +303,10 @@ static void node_free(SpaceLink *sl)
     MEM_freeN(path);
   }
 
-  MEM_SAFE_FREE(snode->runtime);
+  if (snode->runtime) {
+    snode->runtime->linkdrag.reset();
+    MEM_freeN(snode->runtime);
+  }
 }
 
 /* spacetype; init callback */
@@ -534,10 +537,7 @@ static SpaceLink *node_duplicate(SpaceLink *sl)
 
   BLI_duplicatelist(&snoden->treepath, &snode->treepath);
 
-  if (snode->runtime != nullptr) {
-    snoden->runtime = (SpaceNode_Runtime *)MEM_dupallocN(snode->runtime);
-    BLI_listbase_clear(&snoden->runtime->linkdrag);
-  }
+  snoden->runtime = nullptr;
 
   /* NOTE: no need to set node tree user counts,
    * the editor only keeps at least 1 (id_us_ensure_real),