Creating and updating physics objects

WIP

Unless you've set up the physics properties of an entity in the Maratis Editor, physics won't be enabled for that entity. If you want to activate physics for the entity via code, you need two step:

1) create the physics properties;

2) create the real physics object.

The physics context is the what manages the game physics. MScene is using it to prepare all the necessary objects at the start of the scene, but you'll want to use it too if you want to change something in real time.