API: Manager system

[GauGoth-Corp]

• EventManager to centralize game events (callbacks) and their listeners (keys, triggers, collisions, custom events...)
• InputManager to centralize game input handling
• SceneManager to manage multiple game scenes (menus, levels...) and transitions between them
• ResourceManager to centralize the loading and stock of game assets
  • try/except for loading function, "pink" texture as fallback
• TextureManager to manage and stock game textures; texture atlases (=> sprites) and animations
  • define a standard for texture atlases & anims (associated .json files?)
• AudioManager to manage and stock game SFX and music
• UIManager to define specific game UI elements and keep an overall style (fonts, colors...)