Move tempstring off of Graphics, Game, and UtilityClass

Unlike, say, the scriptclass i/j/k stuff, these tempstrings are only
purely visual, and there's no known glitches to manipulate them anyway.
Thus I think it's safe to make this cleanup.

desktop_version/src/UtilityClass.cpp

@@ -132,7 +132,7 @@ std::string UtilityClass::twodigits( int t )
 std::string UtilityClass::timestring( int t )
 {
 	//given a time t in frames, return a time in seconds
-	tempstring = "";
+	std::string tempstring = "";
 	temp = (t - (t % 30)) / 30;
 	if (temp < 60)   //less than one minute
 	{