PartDesign: Add transparent previews

src/Mod/Part/App/AppPart.cpp

@@ -98,6 +98,7 @@
 #include "GeometryIntExtensionPy.h"
 #include "GeometryMigrationExtension.h"
 #include "GeometryStringExtensionPy.h"
+#include "PreviewExtension.h"
 #include "HyperbolaPy.h"
 #include "ImportStep.h"
 #include "LinePy.h"
@@ -435,7 +436,7 @@ PyMOD_INIT_FUNC(Part)
 
     Part::AttachExtension       ::init();
     Part::AttachExtensionPython ::init();
-
+    Part::PreviewExtension      ::init();
     Part::PrismExtension        ::init();
 
     Part::Feature               ::init();