Fix drag curor visibility on Windows.

Fix drag curor visibility on Windows.

QDrag has an API to set both pixmap (QDrag::setPixmap) and drag cursor
(QDrag::setDragCursor): http://qt-project.org/doc/qt-5.0/qdrag.html.

We cannot return from createCursors if there is no pixmap, but we need
to go through also dragCursor and use those if one is set.

Change-Id: If2b2139ad193a4ab5b25c65400c595dc7c33de2c
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>