Node Editor: Add overlay for reroute auto labels

Add an overlay option to automatically display a label on reroute nodes.
This automatic label is propagated through chained reroute nodes and
is based on the socket the reroutes are linked to. If a reroute node is
explicitly labeled, that label is propagated instead.

The automatic label is dimmed to distinguish it from manual labels.

@@ -474,0 +515,4 @@
+  node_sock_label_clear(output);
+}
+
+static void update_reroute_node_auto_labels(bNodeTree *ntree)

@@ -310,6 +310,8 @@ void node_type_size_preset(bNodeType *ntype, eNodeSizePreset size);
 /** \name Node Generic Functions
  * \{ */
 
+void ntree_update_auto_labels(bNodeTree *ntree);

@@ -2064,0 +2064,4 @@
+static void rna_Node_update_node_labels(Main *bmain, Scene *scene, PointerRNA *ptr)
+{
+  bNodeTree *ntree = reinterpret_cast<bNodeTree *>(ptr->owner_id);
+  bNode *node = static_cast<bNode *>(ptr->data);

@@ -474,0 +476,4 @@
+  ntree->ensure_topology_cache();
+  const blender::Span<bNode *> nodes = ntree->toposort_left_to_right();
+
+  for (bNode *reroute : nodes) {

@@ -474,0 +491,4 @@
+      continue;
+    }
+
+    const bNodeSocket *from_sock = linked_sockets.first();

@@ -522,0 +527,4 @@
+  const char *socket_label = blender::bke::nodeSocketLabel(&sock);
+  switch (sock.typeinfo->type) {
+    case SOCK_FLOAT:
+      if (socket_label == StringRef("Value")) {