Bounding Boxes seem to be functional now :)

I added BoundingBoxRenderer.cs for drawing the bounding boxes. Because this class uses GraphicsDevice, I reached that down from Game1.cs over SceneManager.cs... to GameObject.cs. Maybe that could be done better, I didn't know how and at least it's working.
The Bounding Boxes are calculated correctly now (hopefully) and I they can easily be shifted, when their model moves, like I added to ObstacleMoveable.cs