Fix drag and drop rearrangement of nuxeo-data-table elements in Designer layouts

Steps to Reproduce:

• Create a custom document and configure custom layouts
• Create a custom workflow for said document (example)
• Go to Designer to configure workflow layout
• Drag and drop existing elements in view mode to reorder them.
  => First attempt, no red highlighting to show where element will be dropped
  => Second attempt, red highlighting appears, but element is not properly dropped
  => Element "hangs" in a weird overlay
  => Navigating to a pre-existing document layout (discarding any changes) keeps the overlaid undropped element.

See attached designer-layout-bug.mp4 screencast for demonstration.