(svn r6141) -Codechange: introduced DepotID and used it as much as possible

-Codechange: DeleteDepot removes a depot from the pool
-Codechange: DestroyDepot is called by DeleteDepot to remove all things where a depot depends on.
  Last 2 changes to prepare for new pool system. Not pretty now, will be soon.
-Codechange: Removed DoDeleteDepot as it was stupid

openttd.h

@@ -39,6 +39,7 @@ typedef uint16 VehicleID;
 typedef uint16 StationID;
 typedef uint16 TownID;
 typedef uint16 IndustryID;
+typedef uint16 DepotID;
 typedef byte PlayerID;
 typedef byte OrderID;
 typedef byte CargoID;