Game.c: parking slots cannot be selected if an aircraft in STATE_PARKED state is on it.
Other fixes and improvements.
Added some tiles from TILESET2.TIM
+ Added first tutorial level.
* Font now inserts line feed automatically if the next word is too long to fit.
* Gfx.c: added primitive list double buffering in order to gain some performance.
* MapEditor: now airport can be defined inside the tool.
* Game.c: new prototypes GameGetParkingDirection() and GameGetRunwayDirection().
* Slight optimizations into GameRenderLevel().
* Added some comment headers into Gfx.c.
* Timers with no repeat flag were being set to NULL, but this is not actually desired!
* Reenabled transparency for aircraft shadow.
* Deprecated NotificationRequest flags, and replaced by system timer handling.