PhysicsObject physics state update fix

[SoloByte]

Previously the PhysicsObject and CollisionObject would update their state in the open update function even if fixed physics framerate is enabled. The PhysicsObject would update the internal physics state in the normal Update() function instead of the FixedUpdate() function and the CollisionObject would update all the colliders in the Update() function instead of the FixedUpdate() function.

This PR introduces new FixedMode and FixedStep properties in the GameTime class, so that every function always knows if the fixed framerate system is enabled and if the current GameTime instance holds fixed or open timing information.

This PR also includes some fixes in some of the Example scenes that do not properly make use of FixedUpdate(). For instance because of this I found out that the EndlessSpaceExampleScene updates the collision handler twice in each frame instead of only once!

• New FixedMode/FixedStep GameTime properties added.
• Update loop refactored to use new GameTime properties.
• ExampleScenes fixed/updated.