The same BoxShapeParam class is used for two different shape classes
(BoxShape and BoxWaterShape) so it's more natural to move the factory
function to the shape class itself
"Water" is used as a suffix (e.g. a box of water is called "BoxWater",
not "WaterBox").
This naming convention has the benefit of keeping related shape types
next to each other in the source file list.
What we currently call BoxBody, CapsuleBody, etc. is actually
BoxShape, CapsuleShape, etc. (those classes encapsulate Havok shapes)
So this commit renames Body -> Shape and Shape -> ShapeParam.