New IEntity properties: Coordinates (set), Identifier. Loadable entities should...

Documented Game methods and added new ones: AdvanceLevel, Reset, RemoveEntity.
New Map property: Map.Next gets the name of the next map to load.
Parse.Constant change: return type is now nullable and will return null if parsing fails.
Refined triggers/scripts: added trigger options, more consistent behavior, many more functions.
Added key/lock design concept to level1, using Player.Inventory.

git-svn-id: https://bd85.net/svn/cs3505_group@154 92bb83a3-7c8f-8a45-bc97-515c4e399668