particles: use Path rather than Shape

It's confusing that links to Shape go to "Shape QML Type For specifying an area for affectors and emitters." The basic problem in QtQuick is that many types that sound like they ought to be representational are actually items that render in the scene as soon as they are declared. But Paths are purely representational, because we needed them for PathView. Because of that, we also need Shape to render a ShapePath which contains Paths. At first that seemed excessively verbose, but now I think it's good to separate representation from rendering. We should keep going and make QQuickPath the usual type whenever we need to represent the shape of something... such as the area for affectors and emitters. It has a list of pathElements, and it should be closed:true for this purpose.