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/projectexplorer/buildconfiguration.cpp

@@ -309,7 +309,7 @@ NamedWidget *BuildConfiguration::createConfigWidget()
     LayoutBuilder builder(widget);
     for (BaseAspect *aspect : aspects()) {
         if (aspect->isVisible())
-            aspect->addToLayout(builder.startNewRow());
+            aspect->addToLayout(builder.finishRow());
     }
 
     return named;