Add grouping feature (same as in old graphviz VF tool)

[stevebuik]

the persistence JSON already supports groups. In order to add this feature, you will need to add:

1/ the graphviz.js/diagramAsText fn/template will need to add extra graphviz markup to display the groups.

2/ a lightning UI/component to manage the groups. this could use the Lightning duelling lists component. it should probably replace the entity/fields editor when active so that instant UI feedback is possible. It can include a colour picker for each group.