Add a way to disable automatic scaling of QML on iOS Retina devices

When using Qt Quick on a iOS device, there's an automatic scaling of ~2x applied (depending on actual device the factor changes). In QML one operates always in logical pixels.

However it would be nice to have a way to disable this behaviour and use device pixel sizes in QML. Reasons include

• Designers want to use real pixels sizings, telling the developer "put the label at (200, 100)" in device pixels, one doesn't want the nonsense of using 200 * Screen.devicePixelRatio everywhere
• Applications may want to manage HiDPI themselves