App/Gui: introduce temporary document

Add new argument to Application::newDocument() to create a temporary
    document. Also exposed to Python API App.newDocument() with a named
    argument 'temp'.

    The temporary document is marked with status bit 'TempDoc'. The user
    will not be prompt for saving when closing. The undo/redo is disabled.
    The AutoSaver skips it. And the tree view will not show it.
    PropertyXLink allows linking to/from object within a temporary document
    without saving.

src/Gui/Application.cpp

@@ -927,17 +927,25 @@ void Application::onLastWindowClosed(Gui::Document* pcDoc)
             // Call the closing mechanism from Python. This also checks whether pcDoc is the last open document.
             Command::doCommand(Command::Doc, "App.closeDocument(\"%s\")", pcDoc->getDocument()->getName());
             if (!d->activeDocument && d->documents.size()) {
+                Document *gdoc = nullptr;
                 for(auto &v : d->documents) {
+                    if (v.second->getDocument()->testStatus(App::Document::TempDoc))
+                        continue;
+                    else if (!gdoc)
+                        gdoc = v.second;
+
                     Gui::MDIView* view = v.second->getActiveView();
-                    if(view) {
+                    if (view) {
                         setActiveDocument(v.second);
                         getMainWindow()->setActiveWindow(view);
                         return;
                     }
                 }
-                auto gdoc = d->documents.begin()->second;
-                setActiveDocument(gdoc);
-                activateView(View3DInventor::getClassTypeId(),true);
+
+                if (gdoc) {
+                    setActiveDocument(gdoc);
+                    activateView(View3DInventor::getClassTypeId(),true);
+                }
             }
         }
         catch (const Base::Exception& e) {