When adding a cylinder, it was added at the end of the list.
This would make hidden cylinders visible as the new rule is
to only hide unused cylinders at the end of the list.
Therefore, add the cylinder after the last used cylinder,
i.e. before the first hidden cylinder.
This means that the position where the cylinder is added has
to be hidden in the undo command.
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>
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