Do something about the out-of-date and confusing S60 naming in raster pixmap and paint engine classes.

QS60Pixmap and QS60PaintEngine have confusing and in fact wrong names, originating from the days of targetting S60 3.x and 5.0. In the S^3 world these are the Symbian-specific raster pixmap and paint engine implementations which are only used in certain situations (e.g. when painting into a pixmap or image, or forcing the raster graphics system for the app). With QTBUG-19880 QS60PaintEngine is now used as the paint engine for QImage too, therefore some renaming has to be done to avoid further confusion: QS60Pixmap -> QSymbianRasterPixmap, QS60PaintEngine -> QSymbianRasterPaintEngine.

This is a long pending task since the introduction of additional graphics systems on Symbian (OpenVG, OpenGL).