Pad out the string with characters from the right until it is TotalWidth wide. If the string is TotalWidth chars wide or wider already, it is returned unmodified. This function does not truncate strings: see FixedWidthStr to pad and truncate.
The string to pad
The width to pad to
The character to use for padding, by default a space
xf.Str.PadRight("Bear", 10, " ") => "Bear "
xf.Str.PadRight("Bear", 10, "x") => Bearxxxxxx