Show warning dialog if NewGRFs use too many string IDs.

Show when reloading/changing NewGRF config, or starting new game.

src/newgrf_config.cpp

@@ -193,6 +193,8 @@ GRFConfig *_grfconfig;
 GRFConfig *_grfconfig_newgame;
 GRFConfig *_grfconfig_static;
 
+bool _grf_bug_too_many_strings = false;
+
 /**
  * Construct a new GRFError.
  * @param severity The severity of this error.