- Reduce duplication between 'letter-spacing' and 'word-spacing' prose

- Fix error in handling single spacing values
- Define percentages as absolute, and relative to glyph width
- Remove percentages from 'letter-spacing'
- Re-add 'last' value ot 'hanging-punctuation'; it's implemented,
  and it has an effect for right-aligned text.
- Remove language-dependent aspects of 'text-underline-position: auto'

Author: fantasai