disable ref-counting for groups. groups work differently where they are only removed on load if they include no objects.

this was causing groups to be removed by rna if a group's field was cleared and the file was saved (even when it was used elsewhere).

source/blender/makesrna/intern/rna_group.c

@@ -110,6 +110,7 @@ void RNA_def_group(BlenderRNA *brna)
 	srna= RNA_def_struct(brna, "Group", "ID");
 	RNA_def_struct_ui_text(srna, "Group", "Group of Object datablocks");
 	RNA_def_struct_ui_icon(srna, ICON_GROUP);
+	RNA_def_struct_clear_flag(srna, STRUCT_ID_REFCOUNT); /* this is done on save/load in readfile.c, removed if no objects are in the group */
 
 	prop= RNA_def_property(srna, "dupli_offset", PROP_FLOAT, PROP_TRANSLATION);
 	RNA_def_property_float_sdna(prop, NULL, "dupli_ofs");