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/signs_cmd.cpp

@@ -17,6 +17,7 @@
 #include "viewport_kdtree.h"
 #include "window_func.h"
 #include "string_func.h"
+#include "signs_cmd.h"
 
 #include "table/strings.h"