Windows/Direct2D platform plugins: Add Q_DECL_OVERRIDE.

Replace old virtual declarations by Q_DECL_OVERRIDE where
applicable.
Errors discovered: QWindowsScreen::primaryOrientation() did not
overwrite any method, changed to orientation().

Task-number: QTBUG-38858
Task-number: QTBUG-38993
Change-Id: Ie85d5235f3cd35c7a24cac3c1c7693c6508b1009
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>