Support drag and drop of box steps

[waj]

-Allow reordering

• Allow moving in and out of a loop box.

Note: a token with multiple values outside a loop box represents a list, the same token inside a loop represents the iterated value for that table.

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• Imported from BitBucket: https://bitbucket.org/instedd/mbuilder/issue/67
• Originally Reported By: Nicolas di Tada
• Originally Created At: 2014-01-09 18:49:53+00:00