MDL-16423 - big refactor of the way callers interact with the portfolio code

MDL-16423 - big refactor of the way callers interact with the portfolio code

added two new contract methods to the caller class, load_data and expected_callbackargs (static)
this means that the base class is the only place that needs a constructor
and that no data loading happens in the constructor
this in turn means we can check callback argument validity much more lightly

also completely remoted portfolio_add_button function and replaced with a class
as the argument list was getting out of control.  it's now much more readable.