Improve extensibility of the scene, refactor, fixes

[kamil-sita]

Hi!

This pull request aims to:

• improve extensibility of the Scene class:
  • SceneManager now instead uses AbstractScene, meaning that users can provide their own Scene implementation
  • Scene uses method for access internally, meaning that it can be easily extended
• clean up code a bit
  • fixed formatting around some areas
  • moved createLight and createCamera to static util methods - it does not make sense for them to be non-static methods of Scene
  • fixed some typos

One change that I've also added was adding getFirstDirectionalShadowLight() method. I believe that the current code was buggy - if the first DirectionalLight was not DirectionalShadowLight, but the second was, no shadows would be generated!

Let me know what you think about those changes.