(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)

ai.c

@@ -12,6 +12,7 @@
 #include "pathfind.h"
 #include "economy.h"
 #include "airport.h"
+#include "depot.h"
 
 // remove some day perhaps?
 static Player *_cur_ai_player;