Initially, the default section title was swapped within the course format
javascript functions. Though some section titles may be custom and does not
require swapping, in addition they may have links to single section view. This
introduces some callback functions for define what needs to be done in
sections libs and js files.
It allows to define M.course.format.swap_sections and
M.course.format.get_section_selector in the course format javascript file, so
that course dragdrop is aware about perculiar layout and changes that need to
be done when sections are swapped.
Paul Nicholls