QmlApp template: Use new 4.7.1 API to enable qml debugging

QmlApp template: Use new 4.7.1 API to enable qml debugging

Qml debugging is now disabled by default, and has to be enabled
explicitly (b2016bbfc9). This has to be done before the
QDView/QDEngine is instantiated ...

The patch introduces a static create method to QmlApplicationViewer,
so that the setup can be hidden from the users main file.

Reviewed-by: Alessandro Portale