Implement QPainterPath::quadTo() which takes an arbitrary number of control points

As a convenience, add an overload to QPainterPath::quadTo() which takes an end point and a list of control points of any length. The data should be interpreted so that the line between two control points gives the tangent of the curve at the point directly in the middle between the two points.

This is used e.g. for outline glyphs, so code can be simplified in some of the font engines.