Feature request: elements for other graphical primitives

[teunbrand]

The request here is to introduce theme elements that support drawing polygons and points.
There a no cases in ggplot2 itself where this is currently useful, but I think it might benefit the extension community.
Sharing a common definition of what a point or polygon theme element is like prevents mutually exclusive and confusing implementations.
Aside from constructors like element_polygon() and element_point(), I think it is also important to have a root theme element, like theme(point, polygon) that has sensible defaults.