Add a vertical line in the tree column

[laksono]

It is often difficult to keep track of the depth of a particular scope when scrolling up or down across expanded call trees.
A minor visual queue: light grey vertical lines that represented the depth of scope in the lower panel. Something roughly like:

v Sigma Experiment Aggregate Metrics
    v <program root>
    |    v main
    |    |   v foo
    |    |   |   v bar
    |    |   |   |   bum
    |    |   > baz
 
etc.

[blue42u]

This repository has moved to GitLab, please continue this issue there.