[General:006]: Move and reorder

The sequence diagram editor SHALL differentiate between move an element or fragment, that is change its location and shift down, up, left or right any following element without change the semantic order of fragments and lifelines, from reorder fragments or lifelines, that is to be able to change the order of fragments or lifeline by removing it from the current position and inserting it in the new location between two existing fragments or lifelines.

---
### Updated 22 November 2018

The user's intention to re-order elements SHALL be indicated by holding down the `Cmd` key (`Ctrl` on Winux). Absent this modifier, the moving of an element
- SHALL position the element at the intended new location and all subsequent elements in the trace SHALL be moved down in the case that the element is dragged downwards in the diagram
- SHALL position the element only directly below the preceding element in the trace (with padding per #26) and no higher

Note that this implies that dragging an element down the diagram without the keyboard modifier will make space in the diagram by pushing elements down, into which space then new elements may be inserted. It is not possible to make space by pushing elements _up_ the diagram as there is a hard ceiling on the layout (that being the lifeline heads).

As a stretch goal, the pointer cursor MAY be changed to something indicative of semantic re-ordering instead of simply moving when the `Cmd` key is pressed, to indicate to the user that

- [x] re-order when `Cmd` key is pressed
- [x] move following trace to make space for moved element when moving an element down
- [ ] semantic re-ordering cursor