Tagging of items

Also-by: Michael Grammling <m.grammling@external.telekom.de>
Also-by: Andre Fuechsel <a.fuechsel@telekom.de>
Signed-off-by: Alex Tugarev <telekom@tugarev.de>
If this set is mutable, it can be changed at any time from the outside - this is probably not intended here.