Feature/#148 access to parents

Overview of changes:

• Components know their Entity (GetEntity())
• Entities know their Scene (GetScene())
• Systems know their Engine (GetEngine())
• Engine knows the Scene -> the Scene is handled as a shared_ptr, so nobody owns the Scene. (GetScene(), SetScene())
• RenderingSystem does not store the Scene anymore, instead asks Engine
• DefaultEngineSetup() does not need parameters anymore: it initializes the Engine with an empty scene
• Updated viewer to reflect changes
• Fixed integration test (test_opengl_buffer_data_download), as it had been broken for some time (-> #155)

@ademiralp, @mbellgardt