(svn r1817) -Codechange: Moved depot-functions to depot.c

-Codechange: Added wrappers around depot-access (GetDepot no exists)
-Codechange: Made depot-functions a bit more logic (no longer 
GetDepotByTile crashes your game when you request it on a non-depot tile)
-Add: made depots dynamic (yes, 64k depots are possible now)

airport_gui.c

@@ -11,6 +11,7 @@
 #include "vehicle.h"
 #include "station.h"
 #include "airport.h"
+#include "depot.h"
 
 static byte _selected_airport_type;