this avoids a lot of unnecessary (add|remove)_change_listener as
creating temporary Selections do not call that anymore.
Use can choose between a SelectionList which or a DynamicSelectionList
depending on wethear the buffer will be modified or not during the
selections lifetime.
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).
