fixes #277: prevent NPE with checks and a default String
Merge branch 'bug/277-NPE-in-ALKIS-Utils' into dev
 #278 added billing pop-up in Sb_StadtbildPreviewImage

Before a high res image is downloaded the billing pop-up is shown.
 #278 added billing pop-up to SerieAggregationRenderer

The billing-pop-up is shown before one or multiple high res images are
downloaded in Sb_stadtbildserieAggregationRenderer
 #278 added few comments to BillingPopup
 #278 changed logger settings in Sb_stadtbildserieIconFactory
 #278 updated billing.json

Some discounts were changed in the billing.json and the
Stadtbild-product was added.