gscan: auto collapse nodes where appropriate

[oliver-sanders]

GScan displays a hierarchical view of workflow registrations e.g:

foo/
  run1
  run2
bar/
  pub
baz/
  run1

The numbered runs (i.e. the run1, run2, etc) are a new Cylc 8 feature.

For many cases there will only be one run (run1), so displaying this full hierarchy is kinda annoying e.g:

foo/
  run1
bar/
  run1
baz/
  run1
pub/
  run1

To make GScan more compact and to help ease users into the numbered runs we should collapse these nodes somehow (kinda like what GitHub does when a directory contains only one directory e.g. cylc/flow).

foo/run1
bar/run1
baz/run1
pub/run1

Pull requests welcome!

[AaronDCole]

#889

This PR is a POC of how I think this could be fixed, open to feedback or suggestions for improvements or reworks