Multiple scenes

[shikaan]

• Decompose WorldManager in components (View, Scene, SceneGraph)
• Enforce unique_ptrs for managing memory of objects
• Correct spelling in default window name
• Move utils under the lb namespace
• Decouple active and visible objects (i.e., visible can be inactive and viceversa)
• Introduce Scene as top-level construct
  • WorldManager creates and manages the scenes
  • Scenes create and manage Objects
  • SceneGraph is only used for grouping objects (e.g., for rendering, collision check)