RNA: properly tag 'embedded' ID pointers as owned.

This is important at several level, mainly for overrides currently.

Note that this also includes shape keys, since from RNA point of view
those are also 'embedded' (they are not editable, not linkable, ...).

source/blender/makesrna/intern/rna_world.c

@@ -236,6 +236,7 @@ void RNA_def_world(BlenderRNA *brna)
   /* nodes */
   prop = RNA_def_property(srna, "node_tree", PROP_POINTER, PROP_NONE);
   RNA_def_property_pointer_sdna(prop, NULL, "nodetree");
+  RNA_def_property_clear_flag(prop, PROP_PTR_NO_OWNERSHIP);
   RNA_def_property_ui_text(prop, "Node Tree", "Node tree for node based worlds");
 
   prop = RNA_def_property(srna, "use_nodes", PROP_BOOLEAN, PROP_NONE);