Rewrite class structure

[BenCheung0422]

Is your feature request related to a problem? Please describe. As being a part of game engine rewrite, this should also be included. Many behaviours like instancing and member access controls, impact the structure of the game engine a lot. Related to #313. This should be done at first in a game engine rewrite.

Describe the solution you'd like Besides, many game engine-related structures (like getters and setters) should also be standardized to optimize the game engine and reduce maintenance cost. Purpose reorganization (reorganizing structures according to their purposes) is required. Nearly all classes should be able to be instanced to have controls easier. Probably starting a brand-new project is a better idea for maintenance, but that means that Minicraft style might be no longer maintained.