PartGui: avoid code duplication for handing TaskAttacher

A better fix for #0004188

Delay calling setPreviewDisplayMode() in ViewProviderPrimitive::setEdit
also fixed issue of not turning off preview mode if the user cancels
editing because there is already a task dialog active.

src/Mod/PartDesign/Gui/TaskPrimitiveParameters.cpp

@@ -746,6 +746,7 @@ TaskPrimitiveParameters::TaskPrimitiveParameters(ViewProviderPrimitive* Primitiv
     primitive = new TaskBoxPrimitives(PrimitiveView);
     Content.push_back(primitive);
 
+    /*
     // handle visibility automation differently to the default method
     auto customvisfunc = [] (bool opening_not_closing,
                              const std::string &postfix,
@@ -780,6 +781,8 @@ TaskPrimitiveParameters::TaskPrimitiveParameters(ViewProviderPrimitive* Primitiv
         }
     };
     parameter = new PartGui::TaskAttacher(PrimitiveView, nullptr, QString(), tr("Attachment"), customvisfunc);
+    */
+    parameter = new PartGui::TaskAttacher(PrimitiveView, nullptr, QString(), tr("Attachment"));
     Content.push_back(parameter);
 }