Start: Make the Start Page a passive view

Do not attach the Start Page to an existing document as otherwise weird behaviour can be observed:
1. Start FreeCAD
2. Open a project file or data file
3. Open the Start Page
4. Make a change to the document
5. Close the document. You won't be asked to save your change
6. In the tree view the document is still shown because the Start Page is attached to the document
7. Close the Start Page. Now you will be asked to save your change

src/Mod/Start/Gui/StartView.cpp

@@ -100,8 +100,8 @@ gsl::owner<QPushButton*> createNewButton(const NewButton& newButton)
 
 }  // namespace
 
-StartView::StartView(Gui::Document* pcDocument, QWidget* parent)
-    : Gui::MDIView(pcDocument, parent)
+StartView::StartView(QWidget* parent)
+    : Gui::MDIView(nullptr, parent)
     , _contents(new QScrollArea(parent))
     , _newFileLabel {nullptr}
     , _examplesLabel {nullptr}