More points in data source delegate methods

[SteveFortune]

• Start dragging: are we starting drag within a specific area within an item?
• 'Exchanging': were are we dropping ? Problematic when potential drop is on a deletion area and the expected behaviour is for it to snap back

... more examples to come.

[SteveFortune]

• See 'Exchange' is semantically wrong #43 for proposed resolution to exchange vs append ambiguity.
• The start drag data source method should be changed to -(BOOL) canItemAt:(NSIndexPath *)at beDraggedAtPoint:(CGPoint) atPoint inCollection:(UIView<I3Collection> *)collection;