line_fragments now handles 0-width ranges

- comes in handy when using self-closed tags to locate an x/y point
within the Text flow
use NSMagic for RTF parsing
added .groups attr to regex TextMatch objects

- it’s a list of subsidiary TextMatches corresponding to any
parenthetical ‘captures’ in the regex pattern
- the 0’th element corresponds to the entire match (just like the
group() method on re.match objects)