Maybe remove tree of MQTT topics

The actual tree of MQTT topics is currently not beneficial as a) sub-topics are never subscribed to, b) resolution ultimately results in a cached attribute (thus ignoring those structures), and c) the full name needs to be recreated anyway for publishing a message.

There are two solutions to this:

Flatten topic structure

Instead of a tree with parts as inner nods, flatten the tree and only store nodes with full (and maybe overlapping) paths.

Integrate topics in Item

Store topic directly in the Item nonterminal and add bookkeeping for those items with a topic to later find them again if a new message arrives.