Add Migrate support for WorkflowNode data to WorkflowField

This issue describes how to upgrade from a Workflow-module-version 7.x-1.2 or below to a current version 7.x-2.x. You might want to change to Workflow Field (having the workflow status in a 'standard' Field), to take advantage of the standard Field API.

What to do? A migration script is until now not available, but below follows the recipe:
- delete the Workflow directory and recreate the directory from a fresh download;
- run update.php;

During update, the submodule Workflow Node is enabled. The values of Workflow Node and Workflow Field are stored in different places (tables field_MY_WORKFLOW_data vs. workflow_node). So, unless you don't mind the status of current content, you need to do a conversion:
- Enable Workflow Field.
- Create a Workflow Field; link it to an existing Workflow. If you have used that workflow in 1.x, then you need to copy the settings from the Workflow to the Workflow Field. (This is a PITA, in D8 those settings will be in the old spot again...)
- Move/convert/copy the statusses somehow (be creative.. - write some script)
- Disable Workflow Node.
- Rewrite/check your Rules, Actions/custom code.
- Find a way to move all this to your production environment.