Restore ViewObject implementation for all Draft objects

In my tests the code works fine. Using @AIRCAP's instructions,  I have created a 'broken' file from draft_test_objects.FCStd.

src/Mod/Draft/draftobjects/wpproxy.py

@@ -27,10 +27,11 @@
 
 ## \addtogroup draftobjects
 # @{
-import FreeCAD as App
-
 from PySide.QtCore import QT_TRANSLATE_NOOP
 
+import FreeCAD as App
+from draftutils import gui_utils
+
 
 class WorkingPlaneProxy:
     """The Draft working plane proxy object"""
@@ -48,6 +49,11 @@ class WorkingPlaneProxy:
 
         self.Type = "WorkingPlaneProxy"
 
+    def onDocumentRestored(self, obj):
+        gui_utils.restore_view_object(
+            obj, vp_module="view_wpproxy", vp_class="ViewProviderWorkingPlaneProxy", format=False
+        )
+
     def execute(self,obj):
         import Part
         l = 1