Unify semantics of ID classes

For each of QContactId, QOrganizerItemId and QOrganizerCollectionId
the following are true:
 - null values compare less-than non-null values
 - null values compare equal
 - null values convert to empty string representation

This changes also removes the distinction between non-valid and null
QContactId values.

Change-Id: I6d343728873a49a4dfba0109110980c36eb91b90
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>