Utils: Consolidate LayoutBuilder interface a bit

- the var args template for addItems was overkill creating a lot of
  instantiations. Use a temporary list instead.
- allow default constructed LayoutItems to be used for an empty cell,
  avoiding the use of a QLabel with empty text
- add an addRow({...}) overload as convenience shortcut for
  .startNewRow().addItems({...}
- rename startNewRow() to finishRow()

Change-Id: I6d49dacbac3d7acf140ca526884ba1ceeeca2e0d
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/python/pythonrunconfiguration.cpp

@@ -194,7 +194,7 @@ void InterpreterAspect::addToLayout(LayoutBuilder &builder)
         Core::ICore::showOptionsDialog(Constants::C_PYTHONOPTIONS_PAGE_ID);
     });
 
-    builder.addItems(tr("Interpreter"), m_comboBox.data(), manageButton);
+    builder.addItems({tr("Interpreter"), m_comboBox.data(), manageButton});
 }
 
 void InterpreterAspect::updateCurrentInterpreter()