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/Tree.cpp

@@ -2147,6 +2147,8 @@ void TreeWidget::drawRow(QPainter *painter, const QStyleOptionViewItem &options,
 
 void TreeWidget::slotNewDocument(const Gui::Document& Doc, bool isMainDoc)
 {
+    if(Doc.getDocument()->testStatus(App::Document::TempDoc))
+        return;
     DocumentItem* item = new DocumentItem(&Doc, this->rootItem);
     if(isMainDoc)
         this->expandItem(item);