[Workflow editor] Re-enable drag-and-drop support

[ohltyler]

Initial in-progress drag-and-drop support was included when developing the multiple-flow PR #119 . The implementation was not included so we have a clean, incremental history, and don't add unnecessary and unfinished complexity. But it does include a lot of good things to reference when drag-and-drop support is to be added.

Branch with drag-and-drop and specific commit
Branch with drag-and-drop disabled

[dblock]

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