Add fallback parameter to UtilityClass::Int()

If necessary, the caller can provide a fallback to be returned in case
the input given isn't a valid integer, instead of having to duplicate
the is_number() check.

desktop_version/src/UtilityClass.h

@@ -37,7 +37,7 @@ public:
 
     static std::string String(int _v);
 
-    static int Int(const char* str);
+    static int Int(const char* str, int fallback = 0);
 
     static std::string GCString(std::vector<SDL_GameControllerButton> buttons);