Add more functions to handle selections

[merkys]

I am developing a user interface to a geometrical database, and I am happy to use jchempaint as a GUI tool, allowing to visualise a set of user-selected atoms. I am using selectAtom function, which, however, highlights a single atom at time. I would suggest adding a few more functions for more versatile highlighting:

• selectAtoms, which would accept an array indices of atoms to highlight;
• addSelectedAtom, which would add an atom to the current selection;
• removeSelectedAtom, which would remove an atom from the current selection;
• clearSelection, which would un-highlight all atoms.