#858 highlight/selection unification and multiple selections

[gilleain maclean torrance]

highlights and selections are essentially the same - they are subsets of the atom and bond sets that are to be painted differently or annotated.

It would also be good to have multiple selections. It is not necessary (or desirable) to break all the client code that is using selections and highlights at the moment, so any improved system would maintain that interface. However underneath, a list of selections should be maintained, each with an ID that can be used to retrieve them. There would be special highlight and internal selection IDs to preserve existing functions. Perhaps the external selection API could be changed.