[Draft] Enable Transform context menu item

The "Transform" and "Set Colors..." context menu items did not work on
most Draft objects because the View Provider has a setEdit() function,
which overrides any edit action provided at a higher level (e.g. by
Part). This commit checks the mode of the edit, and if it is not zero,
behaves as though the setEdit() function does not exist, allowing Part
to provide the required context menu behavior.

src/Mod/Draft/draftviewproviders/view_base.py

@@ -383,13 +383,19 @@ class ViewProviderDraft(object):
 
         Returns
         -------
-        bool
+        bool or None
             It is `True` if `mode` is 0, and `Draft_Edit` ran successfully.
+            None if mode is not zero.
             It is `False` otherwise.
         """
         if mode == 0 and App.GuiUp: #remove guard after splitting every viewprovider
             Gui.runCommand("Draft_Edit")
             return True
+        elif mode != 0:
+            # Act like this function doesn't even exist, so the command falls back to Part (e.g. in the
+            # case of an unrecognized context menu action)
+            return None 
+
         return False
 
     def unsetEdit(self, vobj, mode=0):