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>
I assume this is a bug - this only makes this.tags unmodifiable, while "tags" is still mutable and thus could be change by the caller.