Core: simplify ILocator interface

Use data members instead of virtual functions for id, displayName and
priority, use Core::Id, not QStrings for id, de-pimpl CommandLocator.

Change-Id: Id8b41f184cb995138b2d76c923d6d3ae02b7e3f5
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: hjk <qthjk@ovi.com>