Replace the old Level:makePhysicsObjects()

The old Level:makePhysicsObjects() would look at all tiles and convert
them into a large amount of rectangles. Not only is this inefficient
but it doesn't have an answer to more complex landscape like slopes.
The new function looks for an object layer called 'Collision' and will
generate rectangles for each rectangle object. In the future, there
will be support for many more types of objects however.