WIP: BKE_lib_query: Add a partial ID usage iterator system.

The idea is to allow iterating over e.g. all ID usages of a node from a
whole nodetree, using the same generic handling as existing 'whole ID'
`foreach_id` code.

This is necessary in some cases wher a sub-data needs to processed
independently from any 'owner ID', e.g. in some copy/paste handling.

This is a pre-requirement for proper fix of nodes copy/paste (see
e.g. #120103).

@@ -1420,0 +1424,4 @@
+ *
+ * See BKE_lib_query.hh and #IDTypeInfo.foreach_id for more details.
+ */
+void node_node_foreach_id(bNode *node, LibraryForeachIDData *data);

@@ -0,0 +290,4 @@
+    }
+    return IDWALK_RET_NOP;
+  };
+  BKELibraryForeachSubdataID().operator()<bNode>(context.test_data.bmain,

@@ -271,2 +271,4 @@
 /**
  * Loop over all of the ID's this data-block links to.
+ *
+ * @param bmain The Main data-base containing `owner_id`, may be null.

@@ -278,1 +285,4 @@
                                  int flag);
+
+/**
+ * Apply `callback` to all ID usages of the data as dfined by `subdata_foreach_id`. Useful to e.g.

@@ -402,0 +411,4 @@
+  BLI_assert((flag & (IDWALK_RECURSE | IDWALK_DO_INTERNAL_RUNTIME_POINTERS |
+                      IDWALK_DO_LIBRARY_POINTER | IDWALK_INCLUDE_UI)) == 0);
+
+  LibraryForeachIDData data;