Visually emphasize dragging a WF operation

[felix-itz]

Currently, when re-ordering WF operations, the elements will be shifted in place. Placeholders shown for the element dragged away and the position inserted could provide better visual feedback. No visual indication of the initiation of the drag process can also cause unintended rearrangement.

opencast-wf-editor-re-arrange2.mp4

desired-wf-editor-re-arrange2.mp4

[felix-itz]

Didn't notice the cursor changes 🙈