Allow column and table deletion

[waj]

When deleting elements that participate in steps, replace those pills with placeholders, like when a pill in a message has been deleted.

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• Imported from BitBucket: https://bitbucket.org/instedd/mbuilder/issue/20
• Originally Reported By: Nicolas di Tada
• Originally Created At: 2013-08-28 18:03:29+00:00