QmlDesigner: Add pixel metric tool bar extension

Change-Id: I65f0ce65113f9d8b4af0ea95a914e2fd4fc2bb07
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
(cherry picked from commit d105b20c7e)
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/coreplugin/manhattanstyle.cpp

@@ -587,6 +587,10 @@ int ManhattanStyle::pixelMetric(PixelMetric metric, const QStyleOption *option,
         if (qobject_cast<const QLineEdit*>(widget) && panelWidget(widget))
             return 1;
         break;
+    case PM_ToolBarExtensionExtent:
+        if (StyleHelper::isQDSTheme())
+            retval = 29;
+        break;
     default:
         break;
     }