Allow selecting and deleting all the edges in the graph that are the result of one edge to or from a subgraph in the DOT source

[magjac]

Currently, such edges can be selected individually, but not deleted because it would involve quite drastic changes to the DOT source, possibly involving cloning nodes and edges, which the user probably does not want.

Ideas

• When selecting an edge in the graph that is the result of one edge to or from a subgraph in the DOT source, select also all other edges that are the result of the same edge in the DOT source.
• When deleting such edges, delete the edge to or from the subgraph in the DOT source.
• The edges could be highlighted with a different color to distinguish them from non-subgraph edges.