Utilize canonical edge data in selected edge info table

[fedarko]

Basically, if we select an edge that connects to a collapsed cluster, its source/target ID displayed in the info table should be the same as if the cluster wasn't collapsed. Right now this information is obtained from the ele.source() and ele.target() methods, which gets the current source/target (i.e. the collapsed cluster's ID) -- not the canonical source/target.

We used to do this by looking at the edge IDs, but when we stopped giving edges IDs (due to SPQR issues with that, I think) I guess this issue came up.

It's not the biggest issue -- since this information is still technically correct, just not super useful -- but I would like to address this in the future.