Introduce a struct to handle docstring <-> QString index translation
This new code allows to get rid of some hardcoded stuff and will be a
foundation for handling of consecutive spaces.
This code can be used in the future for getTextLayout, x2pos, and
pos2x in order to streamline code. Actually some features as the naked
parameter are here to prepare for this second set of changes.
No change intended. The modified code is much clearer, though.