Arrow objects include different types of 2D and 3D arrows, which are not text
objects but are very useful to prepare images containing atomic structures, graphs,
text, etc., for presentations at seminars or school teaching, or for producing
printed images for articles or books. Arrow objects can own text objects (only).
This is useful for example to add an equilibrium constant over a double arrow
symbolising a chemical equilibrium. The low-level engine infra-structure required
to implement arrow objects is fully implemented but everything else is not done yet.