Begin implementing the optional QCPainter backend for QtGraphs 2D. Until QCPainter is in its final place, we can't do the actual integration, but the rendering (and the configuration feature for the 2 different backends) should be doable to some extent already, in preparation for the TP release of QCPainter. 

Current Quick Shapes backend should be the default, and QCPainter backend the optional high-performance replacement (with possibly limited features, like dashed lines).