ContactMethod: add bestName getter

This moves the logic for determining the best name to use away
from the roleData function into its own function. This way we
reduce the logic in roleDat() and make it easy to access this
property if we have the pointer to a ContactMethod.

Change-Id: Id1df8e0c0b40627c5c0f1f8ce922a7142c2076d6
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>