Now that actions can live in multiple attachment points, and may or may

not have the same parent across them, we no longer want to represent

only a single list of children.
The children attribute for GroupAction is now a mapping of

attachment point IDs to lists of children. This is set during render of

the model. By grouping it this way, the action will factor in the

correct children for a given group.
This does not impact how actions are configured when defined. It's

simply the representation to the rendering and view code.

Unit tests pass.
Tested that all menu actions contained the right children.