Add support for sub-attribute callbacks as closures.

[Crell]

Description

Allows the subAttributes() method to return a callback as a closure, rather than a method name string.

Motivation and context

Code in strings is awful.

How has this been tested?

New tests included.

Screenshots (if appropriate)

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)