allow group info to be defined where it is really used, not in a remote group info catalog

Group property contents so far are defined in GroupInfoCatalog. This is remote to where it is actually needed. This has been reworked so by now the group info can be defined right where it is used, namely before the call to addPropertyGroup().

This possibility has been used for MaterialItem already, for other places this has to be done in future commits.

After a while (when reworking the generic UI creation), this will disappear again, but until then it makes the code cleaner and therefore life easier ;-)