unicode-characters.py: better display for combining and control characters

* Update column formatting to handle extended character codes (i.e.
  non-BMP characters) which are longer than 4 characters wide
* Add a column with the Unicode character code (decimal)
* Use Python's unicode_escape format to display characters which are
  likely to break display formatting (i.e. control codes, combining
  diacritics, various spaces, etc.)