Research if we can introduce ECS

Class inheritance based entities system in the engine has many problems. Entity component system may introduce benefits, but this should be researched.

Things to consider:

1. We want to keep the compatibility with the vanilla game assets, e.g. scripts which are unaware about the ECS.
2. We also want to provide a facility for game assets to utilize the ECS and its features, so that modders could benefit.
3. There's so much code tied to another code, hacks, workarounds and tangles in the code, so it's a challenging task.