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/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 {