Reversing group labels if they appear on the left of a molecule

[jblack-mestre]

Issue: #2748

In short, we need to take the label that appears on the left of the molecule, split it into its constituent abbreviations, reverse it, then render that reversed label. The hard part is recognising the abbreviations in the label.

I don't think there is a general algorithm that can be used to do this, instead, taking inspiration from the expand abbreviations functionality in indigo, we could have an xml (.inc) file with all the possible abbreviations that we would recognise. With this we could match the abbreviations in the string and split it properly. (I would implement all this)

In general, what do the maintainers think about this idea? If you have a better way of handling this, please let me know.