Need "Edges" only inspected policy for graphs

[bryevdv]

Questions on Twitter about "edge" only hover tooltip:

https://twitter.com/DataDrivenAngel/status/1324015994759745536

and AFIAK this is not possible, since there is not an inspection for just EdgesOnly

On the assumption that (I think) this is a fairly trivial add, I will tentatively schedule for 2.3

[bryevdv]

OK well this is slightly complicated by the fact that get_selection_manager only returns a single selection manager, and currently for GraphRenderer it arbitrarily returns the node renderer:

  get_selection_manager(): SelectionManager {
    return this.node_renderer.data_source.selection_manager
  }

I'll have to think on how best to generalize things here but @bokeh/dev if there are any suggestions or thoughts, please chime in

[bryevdv]

For purposes of this issue things can probably just be made more specific in the inspection policy here:

    const edge_inspection = graph_view.model.get_selection_manager().get_or_create_inspector(graph_view.edge_view.model)

i.e instead of going through graph_view.model.get_selection_manager actually go through the edge rendere get_selection_manager since that's what is needed. But the general question remains for what (if anything) GraphRenderer.get_selection_manager should do since it has multiple "sub" renderers.