- improved/ cleaned up updateRenderables method of shape3d
- added (back) adding each operation to a shape's list of operations
- experimenting with an alternative "subtract" method that returns
a new instances and keeps the original shapes (subtractee and other
operands internally)
- updated creation operation so that the visual reprentations get removed/added
back correctly when undo/redoing
- generating code from a empty list of operation does not throw an error anymore,
just generates an empty string
- experimenting with an alternate subtraction operation