Better visualisations of sections

[jstockwin]

It would be nice if we could draw large box outlines around elements with each section, to make it easy to see where each section is.

This is a little bit tricky, as sometimes the shape will have to be more complicated than a square, for example:

The problem should be something similar to finding the minimum shape containing all the elements in the section. The problem could be slightly more complicated than the above example if the elements are all over the place, but sections always contain continuous groups of elements so it should be okay.

One issue could be that I think at the moment we order based on the centre of an element. Thus if one element is much smaller than another there could be some overlap which shouldn't be there. I think we will have to cope with it not being perfect, but we should try to do pretty well as we don't want it to be misleading.

Note this is the visualise tool so we're not too interested in performance.