Doomsday Engine

Added: Sky definitions. Rather than define the sky properties for every map, each time in the Map Info definition, instead; use a Sky definition giving it a unique ID which is then used in the Map Info definition to link the two. This allows mod authors to patch just the Sky definitions without needing to touch/create Map Infos.

• Added Sky definitions and updated the Map Info definitions of all games.
• Support for the old approach is retained - if the Sky identifier for a given Map Info is not specified or not found, revert to the old behaviour.