Decouple Stub responsibilities from Boxhead ones

[rich-iannone]

Currently the Boxhead uses the Stub in only one method. If we remove that dependency, it will be much more that the Stub drives the Boxhead. In the rendering code, we look at both the Boxhead and the Stub, when we could instead just use the Boxhead.

There are a few ways to decouple the Stub from the Boxhead:

• _get_effective_number_of_columns() method of Boxhead could instead be replaced use final_columns()
• enumerate where final_columns() should be used in the rendering code