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/ios/iosrunconfiguration.cpp

@@ -319,7 +319,7 @@ void IosDeviceTypeAspect::addToLayout(LayoutBuilder &builder)
 
     m_deviceTypeLabel = new QLabel(IosRunConfiguration::tr("Device type:"));
 
-    builder.addItems(m_deviceTypeLabel, m_deviceTypeComboBox);
+    builder.addItems({m_deviceTypeLabel, m_deviceTypeComboBox});
 
     updateValues();