HexToASCII: display information about invisible characters

- Reduce the number of entered characters to 128, since we can't be sure
  of the display width any more.
- Show "invisible characters" (unprintable + non-space whitespace) in a
  special format

I am unsure if the styling is done appropriately or will look as nice as I think on the results page.

Also, there is the localization issue with the description text. I am not sure how big a deal this is, given that it is ASCII.  If it's a big problem, I could always go to the simpler abbreviation-only.