Generalize showOrigin function

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=46. Original issue from byor...@gmail.com on September 3, 2011, 07:05:03 PM UTC)

The showOrigin function (in Diagrams.TwoD.Model) ought to be generalized to take a few more options to control things like the size and color of the dot. No need to go overboard -- after all, it's quite easy to just use atop if you want something more general -- but it would be nice if, say, showOrigin didn't become useless when trying to visualize the origin of something red.

[fryguybob]

(Imported. Original comment by byor...@gmail.com on November 3, 2011, 09:02:08 PM UTC)

Implemented by Ian Ross.