QmlPuppet: Enabling style support

If a style is activated by setting QT_QUICK_CONTROLS_STYLE we have
to use QGuiApplication instead of QApplication.

Change-Id: I138a4fb75508f989f97b207a7024a426fa379f9e
Reviewed-by: J-P Nurmi <jpnurmi@theqtcompany.com>
Reviewed-by: Tim Jenssen <tim.jenssen@digia.com>