PyfaceColor traits should have an allow_none option

This has not been merged yet, but rather than adding to the PR, it probably makes sense to add options to have these traits hold a `None` value as with `Instance` traits. The default should be `False`. When `allow_none` is `True`, if no other default is supplied it should be `None`, as with an instance trait.

This makes it easier for a class to say they have no preference about a color choice.

There may be a reasonable argument to also do this for `PyfaceFont` trait type as well.