This permits to fix a bug in BufferIterator::upgrade, replaced by
BufferIterator::on_insert and BufferIterator::on_erase. ModificationListener
was only used to updating iterators anyway.
Instead of a big std::string, buffer now store it's content in a
list of lines. In order to achieve O(log(n)) random access, lines
contains both their content and their offset since the start of
the file, making binary search usable.
BufferIterator now have a LineAndColumn coordinate into the buffer
instead of an offset so that access is still O(1).
