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NOTICE: This is a work-in-progress...untested
NEW Added YML workflow export logic to hook-into existing workflow creation logic

- Added WorkflowDefinitionExporter class, uses existing YML parsing logic
- Added separate formatter class to allow for easy addition of other export formats
- Add YML template with SS template variables and logic
- Added placeholder methods to allow for user/group data to be exported
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- Download works, but not from gridfield UI
- Correct workflow is exported
- Added Users and Groups to export for Actions & Transitions
- Download prompt now works from GridField
Needed to modify $url_handlers becuase the "Add new Definition" button
failed. This now works, as does the export logic. "Export to CSV" still
fails though..
All buttons work again
- Ensured template var was escaped so it was generated properly in
  exported template
W.I.P added UploadField, but files are not being associated correctly.
Perhaps, a WorkflowDefinition needs saving _first_ before an upload file
can be associated with it...?
W.I.P:
- Upload works OK for YML files
- Added temporary icon for 'export' action on GridField
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- 1st attempt at 'merging' uploaded YAML with that in _config
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- Added DataExtension for File to cater for onAfterUpload() logic
- We're going to ensure this all works by just hard-out copying YAML
  imports into _config and see what happens...

*BOOM*