implement a color mapper class

implement a color mapper class
So I was deeply inspired by my habit of using nested python dictionaries
This provides the semantics of a hierarchy of 3 different colors.
A generic default, a per annotation "type" default, and an "instance" default.

it does assume there's only those 3 levels and nothing else, though it
probably is possible to make it more recursive and capable of supporting
arbitrary depths. However I don't need that.