Implement multiplicative Copy Scale and make it the new default.

Scale is a multiplicative quantity, so adding it doesn't make sense.
However, for backward compatibility reasons, and in case somebody
actually desires the old additive behavior, the old way remains as
an option.

Without this change the only way to properly combine scale is via
parenting or the complicated Transformation constraint.

The new mode is turned on by a flag for file compatibility, but the
RNA option is reversed so that the new behavior feels more default.

Reviewers: aligorith

Differential Revision: https://developer.blender.org/D3558

release/scripts/startup/bl_ui/properties_constraint.py

@@ -420,7 +420,11 @@ class ConstraintButtonsPanel:
         row.prop(con, "use_y", text="Y")
         row.prop(con, "use_z", text="Z")
 
-        layout.prop(con, "use_offset")
+        row= layout.row()
+        row.prop(con, "use_offset")
+        row = row.row()
+        row.active = con.use_offset
+        row.prop(con, "use_add")
 
         self.space_template(layout, con)