Improve typing of tags in the Footprint class

Store these internally in Footprint as a list of strings, and concatenate them at the file output step.

This makes it easier to append new values to the list because the list allows append/extend semantics.

Also provide a a @Property getter and the setter, but keep the old accessor for now.

Update a unit tests with a demo of how these can be used.

---

This is laying some groundwork to unify tag handling across various scripts as an extension to the declarative definition work in !910 (merged). But it doesn't have a dependency on it (later work will)