added the option 'add molecule to group'

- added the option 'add molecule to group' to the
  molecule menu
- implemented the function 'MolconvWindow::DuplicateMolecule()'
- added vectors of groups and stacks to the class MolconvWindow
added a context menu to the molecule list

- this is only a very crude proof-of-concept dummy context menu
added the function 'MoleculeSettings::setValues()'

- this function will set the values of the spinboxes according to the
  properties of the newly added (or selected) molecule