Primitive form of ctrl-objectcopy, bufferlength-simpleundo

Added bufferlength-cut-undo.
Had to change interactive-bond+atom-placement to true undo.
Works only for bonds and atoms, and ignores buffers!
Letting go the ctrl key doesn't remove the shadow, because no restoreundo
command is called then.

Signed-off-by: Lennard Wasserthal <Wasserthal@nefkom.net>