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/qmlprojectmanager/qmlmainfileaspect.cpp

@@ -84,7 +84,7 @@ void QmlMainFileAspect::addToLayout(Utils::LayoutBuilder &builder)
     connect(m_fileListCombo, QOverload<int>::of(&QComboBox::activated),
             this, &QmlMainFileAspect::setMainScript);
 
-    builder.addItems(tr("Main QML file:"), m_fileListCombo.data());
+    builder.addItems({tr("Main QML file:"), m_fileListCombo.data()});
 }
 
 void QmlMainFileAspect::toMap(QVariantMap &map) const