"Set Author From Debug Token" dialog list debug tokens
from Signing manager too.

Fixing BB Device Config Widget to display debug token that is not
registered in Keys option dialog yet.

Change-Id: Ib5775d08e3ce129bee0d03de6cab992a90220fd3
Reviewed-by: Tobias Nätterlund <tobias.naetterlund@kdab.com>
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>