Add UtilityClass::Int()

This is simply a wrapper function around SDL_atoi(), because SDL_atoi()
could call libc atoi(), and whether or not invalid input passed into the
libc atoi() is undefined behavior depends on the given libc. So it's
safer to just add a wrapper function that checks that the string given
isn't bogus.

desktop_version/src/UtilityClass.h

@@ -37,6 +37,8 @@ public:
 
     static std::string String(int _v);
 
+    static int Int(const char* str);
+
     static std::string GCString(std::vector<SDL_GameControllerButton> buttons);
 
     std::string twodigits(int t);