Introduce nativeOrientation() to QPlatformScreen

Introduce nativeOrientation() to QPlatformScreen

Now QPlatformScreen has a nativeOrientation() method which default
implementation return Qt::PrimaryOrientation. Its purpose is to return
the native orientation of the device (e.g. when the device logo is right
up).

Only the QPA backend can know for sure what this native orientation is,
so allow QPA backends to override it with a finer result.

Change-Id: I3bef14acb6b8568e64398ff919d9cb4461a2c9cc
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>