Rename referrables to game objects

**What needs to be cleaned up?**
Referrables should be called game objects instead, as originally intended. Rename IReferrable to IGameObject and update any comments talking about referrables to say game objects instead.

**How will this benefit us?**
Game objects exist within a game, while mod objects exist within a mod. They're both technically referrable, just in different ways. Just clarifying the similarities and differences, and avoiding made-up words that have to be defined for anyone new working on the code.

**What potential drawbacks are there to making this change?**
Regression bugs and wasted time. Porting to Unity could be confusing with two types of game objects. (Maybe call them game/mod entities instead?)

**Related issues**
N/A