add simple string to integer conversion function

Just a basic one in C4Script for now.

planet/System.ocg/String.c

@@ -50,3 +50,19 @@ global func CharIsUpperCase(int char)
 {
 	return Inside(char, 65, 90);
 }
+
+// Converts a string to an integer if it consists of digits only.
+global func StringToInteger(string str)
+{
+	var integer = 0;
+	var power = 0;
+	for (var index = GetLength(str) - 1; index >= 0; index--)
+	{
+		var char = GetChar(str, index);
+		if (!CharIsDigit(char))
+			return nil;
+		integer += (char - 48) * 10**power;
+		power++;	
+	}
+	return integer;
+}