Allow manually ordering nodes within a layer

[keyserj]

Is your feature request related to a problem? Please describe.
Sometimes the diagram is easier to understand when certain nodes are next to each other; for example:

• when nodes of a type are next to each other (layout tries to do this but doesn't guarantee it)
• when nodes are related to each other (e.g. in this topic, implementation criteria should be grouped, post-success criteria should be grouped)
• when an order is implied (steps in a process e.g. steps that refugees go through as new arrivals to US)

Describe the solution you'd like

• be able to drag a node over the left or right half of another node in the same layer to guarantee the order between those two nodes (maybe need some intersection logic like seen here)
• probably should save this order across sessions/refreshing
• this order could also be used for the criteria table, but if UI for dragging nodes in the table is hard, doesn't need to be added now

Questions to answer

Describe alternatives you've considered

Additional context

Technical ideas

• should be able to determine the layer by checking nodes with equal Y position values
• maybe track an order per node that's set when the dragging is done
• not sure best algo to assign orders, but might be good enough to just assign the current X values as the order (this way, if two other nodes in the layer were ordered at some point, there's a chance that they remain ordered as intended)
• use order to set elkjs's layout [ position: x, 0 ] on the nodes (see how it's set here and with this tool)

Technical Questions

• Separate table for view-specific things? Or just put order on the node table?