Create a PathRectangle component for Qt Quick Shapes

The Rectangle component has a specific, optimized shader, and is lacking support for certain features such as different stroke styles, different gradients (it only supports vertical and horizontal linear gradients).

For advanced use cases like this, we have Qt Quick Shapes, but it currently requires creating the rectangle manually from line segments. This becomes especially cumbersome if you want to use the rounded corners feature of the rectangle. Having a PathRectangle composed path feature would be a good convenience and would make it easier to point people to Qt Quick Shapes when they discover the limitations of the Rectangle type.

(Once added, it might also be possible to support advanced features in Rectangle by building on top of this, e.g. if you set a QtQuick.Shapes.LinearGradient on the rectangle, it could internally build on QQuadPath and use the curve renderer)