This allows compositing any two templates, not just filters. The main advantage is that by being a little clever, & because templates follow a fairly simple format, we can do some fancy things without hassle.
As an example, this applies the 'mercenary' tech automatically on the Athen mercenary templates.
While not as flexible as D3800, this change alone would allow us to cleanup a _lot_ of stuff in our templates, e.g.:
- The build entities
- Civ-specific duplication (apply a generic athen| template)
- would allow making the 'archer' or the 'cavalry' bit also applicable.