Make copy / cut / paste selection-aware
Make copy / cut / paste selection-aware

- Only copy selected atoms
- Make sure to only clear selected atoms with a cut
- Only select the new atoms after a paste

Signed-off-by: Geoff Hutchison <9cf35bacc9f8512ca007341c9e919cf5dc03e0c7@gmail.com>

OpenChemistry/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs,ghutchis/avogadrolibs,OpenChemistry/avogadrolibs