R-group support

rdEditor now has support for showing and editing dummy atoms (i.e. R-groups) thanks to a contribution from @jacktday

A new atomtype "R" with atomnumber 0 has been added, and the number can be changed via the "A#" action which can set the atomMapNumber of atoms. The R-groups support save to molfile, and in SMILES they are supported as * atoms. The "A#" can also be used to edit the atommapnumbers of regular atoms.

Also, some icons missing from the previous release has been added.