World refactor and grass

The World class has been refactored: I separated sky and terrains (vegetation was already separated, but the class needs to be named VegetationSystem instead of PagedGeometrySystem). There are now SkySystem which uses Caelum and TerrainSystem (with the TerrainSystemHeightFunction namespace for the height function used by PagedGeometry) which uses ETM. The world was loaded from a XML file taken from the resources, but I've seen it's too slow to parse - so I used a INI-style file which I can read with Ogre::ConfigFile class. This file contains all the information about stage name, author(s), scene (camera position only at the moment), whether to load grass and terrain.