Improve the BigExplosion Class

[fvultier]

If a Pawn is destroyed a big explosion is displayed. The BigExplosion class is responsible for this effect. Unfortunately this class contains too much hard coded stuff. For example the links to the meshes for the explosion chunks are written in the C++ code. It would be better to define these paths in XML. That way we could define different explosion effects for different Spaceships/Pawns. If I destroy a wooden box there shouldn't be metal pieces flying away …