[Lens] Drag color stop colors/values to re-order them

[ghudgins]

Describe the feature:
In the color by value editor, be able to drag and drop the stop colors (values stay where they are, but we should talk about this behaviour)

Describe a specific use case for the feature:
When I am editing my color stops
I need to drag/drop the color sample (little square) I've already defined and place it in any other position within the color sequence
So I can avoid spending few clicks for copy-pasting the current color into a different color sample

[elasticmachine]

Pinging @elastic/kibana-vis-editors @elastic/kibana-vis-editors-external (Team:VisEditors)

[ghudgins]

this no longer makes sense with the latest designs