ItemsComponentModel has obtuse implementation

[oliverfoster]

Subject of the issue/enhancement/features

The way the ItemsComponentModel is written with regards to _items breaks backward compatibility with existing functionality unnecessarily.
_items is replaced by a Collection of Models which makes any existing code relying on _items being an array of objects stop working at this line
toJSON is then overridden to compensate, at this line

Expected behaviour

_items should return an array of objects.
_children should represent the collection, as with the other models.

[moloko]

I think this is all done now?