Fix mapping between screen orientations and rotation angles.

The existing mapping between Qt::ScreenOrientation values and rotation angles is wrong.
This patch fixes it by simply using the existing function for this from Qt.

E.g.:
If primary angle is Landscape and the given orientation is Portrait, the resulting rotation angle is 90, not 270 as in that switch. Documentation for Qt::PortraitOrientation says it is " rotated 90 degree clockwise relative to landscape."