feat: scene state

[Gugustinette]

Add some utilities for loading a scene :

• A scene should have a state

  export enum FSceneState {
    RUNNING = 'Running',
    LOADING = 'Loading',
    PAUSED = 'Paused'
  }

• It is RUNNING by default
• Pause/Resume workflow :
  • The scene can be paused and resumed with pause and resume methods
  • Paused scene stop computing every frame on the main loop and stop firing collisions events
• Loading workflow :
  • LOADING has technically the same effect as PAUSED
  • If a lockLoading method is used on the scene, it turns into the LOADING state
  • Components can be safely added to the scene
  • unlockLoading can be called, which will wait for all the components to be loaded before starting the main loop again
  • The scene goes back to RUNNING