- eliminate value_default
- make value private (-> _value)
- add value() method

- avoid unnecessary internal usage of desktop coordinates, e.g.
  in SpellCheck::compareTextBboxes
- document whether a function argument is in document  or desktop
  coordinates, e.g. for SPDocument::getItemsInBox

Closes #340

Call this->setAttribute(...) directly instead of
this->getRepr()->setAttribute(...). This avoids clutter and mental indirection.

See
http://wiki.inkscape.org/wiki/index.php?title=Refactoring_projects&oldid=112116

and https://en.wikipedia.org/wiki/Law_of_Demeter .

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