all core plugins: update to remove references to deprecated functionality

[moloko]

Mostly to switch over to the new a11y/notify APIs

At the same time also switch over any instances of model.get('_children') to model.getChildren() and model.get('_parent') to model.getParent().

Also start using ES6 where this would improve the code (n.b. this requires a FW dependency bump to >=5.3)