Revert change to QContactRelationship participant types

A earlier change converted QContactRelationship first and second
properties to use QContact rather than QContactId representations.
The motivation for this change is unclear, but it resulted in
undesirable complication to the interface without obvious benefits.

This change restores the original ID-based representation for
relationships.

Change-Id: Iaeff907d11efcda0ef3ba3161ee0c3e760b0dd1a
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>