Add per-corner Rectangle radius property

Rectangle currently has a radius property that acts on all four corners in the same way. There are designs that call for rectangles where only one or two corners are rounded. These currently require quite a bit of composing to get right. It would perhaps be beneficial if Rectangle would gain a `radii` grouped property, with sub-properties for all for corners individually. The current `radius` property can act as a default for all four, just like `anchors.margins` is the default value for all the other margin values that exist in anchors.