Make QT_DEVICE_PIXEL_RATIO an alias to QT_SCREEN_SCALE_FACTORS for backwards compatibility

QT_DEVICE_PIXEL_RATIO is actively used by downstream. KDE uses it in it's screen scaling configuration on many systems.

In 5.6, QT_DEVICE_PIXEL_RATIO behaviour was changed and it no longer produces the expected results.

Setting QT_SCREEN_SCALE_FACTORS=2 produces the same visual result as setting QT_DEVICE_PIXEL_RATIO=2 did before, and as the user would expect from QT_DEVICE_PIXEL_RATIO=2 (or from using KDE configuration).

Perhaps it would make sense to offer a grace period (e.g. 5.6 + 5.7) and print a deprecation warning if QT_DEVICE_PIXEL_RATIO is detected, but actually use that value for QT_SCREEN_SCALE_FACTORS if that was not set explicitly?