Support for higher order bonds

[matterhorn103]

Currently Avogadro supports creating and displaying single, double, and triple bonds. It would be nice to extend this to higher bond orders – up to 6/sextuple is known – to permit visualization and preparation of images of transition metal complexes containing quadruple+ bonds.

Given their relatively infrequent occurrence it makes little sense to have bonds cycle through them when clicked with the Draw Tool, so that should be kept as is. But it would be useful if they were available via the bond order drop-down in the Draw pane. And it would also be good if they would be displayed when read from a cjson file.

[avo-bot]

This issue has been mentioned on Avogadro Discussion. There might be relevant details there:

https://discuss.avogadro.cc/t/handling-of-bond-order/5268/1