QCharAttributes: add wordStart/wordEnd flags

QCharAttributes: add wordStart/wordEnd flags

A simple heuristic is used to detect the word beginning and ending by
looking at the word break property value of surrounding characters.
This behaves better than the white-spaces based implementation used before
and makes it possible to tailor the default algorithm for complex scripts.

BIG FAT WARNING: The QCharAttributes buffer now has to have a length
                 of string length + 1 for the flags at end of text.

Task-Id: QTBUG-6498

Change-Id: I5589b191ffde6a50d2af0c14a00430d3852c67b4
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>