Add function to get GRF ID of string ID

src/newgrf_text.cpp

@@ -1078,3 +1078,13 @@ uint RemapNewGRFStringControlCode(uint scc, char *buf_start, char **buff, const
 			return 0;
 	}
 }
+
+uint32 GetStringGRFID(StringID string)
+{
+	switch (GetStringTab(string)) {
+		case TEXT_TAB_NEWGRF_START:
+			return _grf_text[GetStringIndex(string)].grfid;
+		default:
+			return 0;
+	}
+}

src/strings_func.h

@@ -174,6 +174,7 @@ extern StringParameters _global_string_params;
 char *GetString(char *buffr, StringID string, const char *last);
 char *GetStringWithArgs(char *buffr, StringID string, StringParameters *args, const char *last, uint case_index = 0, bool game_script = false);
 const char *GetStringPtr(StringID string);
+uint32 GetStringGRFID(StringID string);
 
 uint ConvertKmhishSpeedToDisplaySpeed(uint speed);
 uint ConvertDisplaySpeedToKmhishSpeed(uint speed);