Protein deletion is very slow

[smlpt]

Protein objects are made up of several nested partial curve objects. When deleting the protein from the scene in the inspector, it takes almost 8 seconds to delete the protein, using the default protein from Add > Protein from PDB ID.

I think this is caused by the fact that the scene gets updated after every delete operation of every single partial curve. Maybe it would be faster when scene updates are disabled while child objects are being deleted?

[smlpt]

This has been fixed with #587