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/webassembly/webassemblyrunconfigurationaspects.cpp

@@ -91,7 +91,7 @@ void WebBrowserSelectionAspect::addToLayout(LayoutBuilder &builder)
                 m_currentBrowser = selectedBrowser;
                 emit changed();
             });
-    builder.addItems(tr("Web browser:"), m_webBrowserComboBox);
+    builder.addItems({tr("Web browser:"), m_webBrowserComboBox});
 }
 
 void WebBrowserSelectionAspect::fromMap(const QVariantMap &map)