Split codebase into modules, e.g. `core`, `game`, `gpu`, `gui` etc

[congard]

Preliminary list:

• core - e.g. platform.h
• gpu - gpu-related stuff, e.g.:
  • buffers
  • textures
  • context
  • gl.h
  • etc
• gui - widgets, painter
• window
• lua - basic Lua scripting support
• math - curves, points, Color
• game - ECS-based part
  • Model
  • Shape
  • World
  • Animations
• io
  • IOProvider
  • IOSystem
  • Log, Logger