Codechange: Let the compile generate the master command table out of templated command traits.

This is using a non-intrusive type-traits like templated system, which
allows compile-time validation that the command table and the command
enum match up.

src/train_cmd.cpp

@@ -34,9 +34,10 @@
 #include "zoom_func.h"
 #include "newgrf_debug.h"
 #include "framerate_type.h"
+#include "train_cmd.h"
 
 #include "table/strings.h"
-#include "table/train_cmd.h"
+#include "table/train_sprites.h"
 
 #include "safeguards.h"