(svn r2256) - Fix: Trains cannot find a depot when they are in a tunnel. (glx)

- Add: GetVehicleTrackdir() helper function.
- Codechange: Moved SortStruct from vehicle_gui.h to ttd.h, so the dependency from vehicle.h on vehicle_gui.h could be removed.
- Codechange: Typedeffed the VehicleTypes struct so it can be used as the type for Vehicle.type instead of "byte".
- Codechange: Removed prototype for VehicleSorter(), which had no implementation anymore and was never called.

ttd.h

@@ -25,6 +25,15 @@ typedef struct Pair {
 	int b;
 } Pair;
 
+/**
+ * Is used as a general sortable struct (using qsort and friends). Is used for
+ * sorting vehicles and stations at the moment
+ */
+typedef struct SortStruct {
+	uint32	index;
+	byte		owner;
+} SortStruct;
+
 typedef struct YearMonthDay {
 	int year, month, day;
 } YearMonthDay;