(svn r3218) -Feature: Multiheaded train engines will now stay in the same train

This means that any user attempt to remove a rear engine will tell the user to move the front engine instead
	This fixes the assert when moving multiheaded engines (introduced in r3144)
	Note: to make old savegames use this feature, some engines might be turned around in order to link engines in pairs

-Codechange: train subtype is now a bitmask
	This allows fast access to info like if it is a wagon or engine and if it is in front and so on
	Note: savegame version bump

order_gui.c

@@ -17,6 +17,7 @@
 #include "viewport.h"
 #include "depot.h"
 #include "waypoint.h"
+#include "train.h"
 
 static int OrderGetSel(const Window* w)
 {
@@ -282,9 +283,9 @@ static bool HandleOrderVehClick(const Vehicle* v, const Vehicle* u, Window* w)
 {
 	if (u->type != v->type) return false;
 
-	if (u->type == VEH_Train && u->subtype != TS_Front_Engine) {
+	if (u->type == VEH_Train && !IsFrontEngine(u)) {
 		u = GetFirstVehicleInChain(u);
-		if (u->subtype != TS_Front_Engine) return false;
+		if (!IsFrontEngine(u)) return false;
 	}
 
 	// v is vehicle getting orders. Only copy/clone orders if vehicle doesn't have any orders yet