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);
 }
 

src/Mod/PartDesign/Gui/ViewProviderPrimitive.cpp

@@ -81,8 +81,6 @@ void ViewProviderPrimitive::setupContextMenu(QMenu* menu, QObject* receiver, con
 bool ViewProviderPrimitive::setEdit(int ModNum)
 {
     if (ModNum == ViewProvider::Default ) {
-        setPreviewDisplayMode(true);
-
         // When double-clicking on the item for this fillet the
         // object unsets and sets its edit mode without closing
         // the task panel
@@ -114,6 +112,8 @@ bool ViewProviderPrimitive::setEdit(int ModNum)
         else
             Gui::Control().showDialog(new TaskPrimitiveParameters(this));
 
+        setPreviewDisplayMode(true);
+
         return true;
     }
     else {