#73 Reorder blocks of diagrams

[Peter Becker]

Requested by Tim Kaiser:

he wants to move whole blocks of diagrams up or down in
the list of diagrams to reorder them, instead just one.
I think it would mean we have to introduce multiple
selection in the list and then figure out which of the
selected diagrams to display (or display a diagram only
if only one is selected). The up and down buttons would
need some more logic to figure out the boundary conditions.