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/cmakeprojectmanager/cmakebuildstep.cpp

@@ -458,7 +458,7 @@ BuildStepConfigWidget *CMakeBuildStep::createConfigWidget()
     auto frame = ItemViewFind::createSearchableWrapper(buildTargetsView,
                                                        ItemViewFind::LightColored);
 
-    builder.startNewRow().addItems(tr("Targets:"), frame);
+    builder.addRow({new QLabel(tr("Targets:")), frame});
 
     updateDetails();