Method insert. Requests that the receiver insert object(s) making up a new row at
the specified position. The receiver returns the actual position where the insert
occured (that doesn't have to be the same as the requested position).
positionHint - The user's current position in the user interface relative to the
beginning of the collection (0-based).
the actual position of the new object or -1 if a new object could
not be inserted.