Nodes: expose ordering of links in multi input socket in Python

This fixes #116096 and is based on #115249 with some changes.

This patch contains three changes:
* `NodeSocket.links` in the Python API gives the links in the correct order.
* `NodeLink.multi_input_sort_id` gives read-only access to an id that's used
  for sorting. Note that for historical reasons the highest id is the first link.
* `NodeLink.swap_multi_input_sort_id(other_link)` provides a safe way to
  change the order of links connected to a multi-input socket.

Co-authored-by: Adrian Bibby Walther <adrianbibbywalther@gmail.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/118987

scripts/modules/bpy_types.py

@@ -1258,10 +1258,15 @@ class NodeSocket(StructRNA, metaclass=RNAMetaPropGroup):
         List of node links from or to this socket.
 
         .. note:: Takes ``O(len(nodetree.links))`` time."""
-        return tuple(
-            link for link in self.id_data.links
-            if (link.from_socket == self or
-                link.to_socket == self))
+        links = (link for link in self.id_data.links
+                 if self in (link.from_socket, link.to_socket))
+
+        if not self.is_output:
+            links = sorted(links,
+                           key=lambda link: link.multi_input_sort_id,
+                           reverse=True)
+
+        return tuple(links)
 
 
 class NodeTreeInterfaceItem(StructRNA):