Add QUuid to Entity

The QUuid will allow us to refer to any entity in the system and to also
use the QUuid to search for resources corresponding to a given entity
in the upcoming resource managers.

Change-Id: If154e94c6adb1441d6bbf055c64059cd86a5a4be
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>