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/qbsprojectmanager/qbsinstallstep.cpp

@@ -195,16 +195,13 @@ BuildStepConfigWidget *QbsInstallStep::createConfigWidget()
     commandLineTextEdit->setMinimumHeight(QFontMetrics(widget->font()).height() * 8);
 
     LayoutBuilder builder(widget);
-    builder.addItems(new QLabel(tr("Install root:")), installRootValueLabel);
-
-    builder.startNewRow();
-    builder.addItem(new QLabel(tr("Flags:")));
+    builder.addRow({tr("Install root:"), installRootValueLabel});
+    builder.addRow(tr("Flags:"));
     m_dryRun->addToLayout(builder);
     m_keepGoing->addToLayout(builder);
     m_cleanInstallRoot->addToLayout(builder);
 
-    builder.startNewRow();
-    builder.addItems(commandLineKeyLabel, commandLineTextEdit);
+    builder.addRow({commandLineKeyLabel, commandLineTextEdit});
 
     const auto updateState = [this, commandLineTextEdit, installRootValueLabel] {
         installRootValueLabel->setText(installRoot());