BUG: fix the SaveAsDefault in ColorMapEditor

When clicking on "Save as default" in the ColorMapEditor,
a warning said the property ScalarOpacityFunction was not found.
The property was searched in a bad proxy. Now the property is found.

Change-Id: Idfd3434f5835fe7faccf0fec9433df891c1ac740
Merge topic 'bug_color_save_as_default' into next

3b4f9ed9 BUG: fix the SaveAsDefault in ColorMapEditor