generalize ModelController, add Model.type

[#41923323]

There is now a general-purpose ModelController that contains
almost all of the code that was in the MD2D ModelController

  common/controllers/model-controller

The MD2D is now greatly simplified and given a more generic name:

  md2d/controllers/controller

Additionally the Model function returned by:

  Model = require('md2d/models/modeler')

now has a type property.

  Model.type => "md2d"