This adds a new mode to modifications called "tokens" which allows clever token parsing.
Technologies, auras and modifiers in general can use this to switch out, add or delete tokens dynamically.
----
Implemented use cases:
- Changing production queue and builder entities at runtime.
- Could be used for phase upgrades / unit upgrades.
----
Future use cases (will be implemented later, as they are trickier):
- Changing garrisonable entities.
- Changing healable classes.
- Changing attackable classes.
- Changing auras dynamically.
- Changing Identity stuff
- Should be extendable to do fancy-pants props switching on tech upgrades, ultimately.
- That might make it possible to simulate "promotion" without actually promoting, too.