(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

viewport.c

@@ -19,6 +19,7 @@
 #include "signs.h"
 #include "waypoint.h"
 #include "variables.h"
+#include "train.h"
 
 #define VIEWPORT_DRAW_MEM (65536 * 2)
 
@@ -1700,7 +1701,7 @@ static void CheckClickOnLandscape(const ViewPort *vp, int x, int y)
 
 static void SafeShowTrainViewWindow(const Vehicle* v)
 {
-  if (v->subtype != TS_Front_Engine) v = GetFirstVehicleInChain(v);
+  if (!IsFrontEngine(v)) v = GetFirstVehicleInChain(v);
   ShowTrainViewWindow(v);
 }