[TreeView] Have down arrow go to first child

[imnasnainaec]

Up arrow goes to parent, side arrows go to siblings, down arrow only works when there's only one child.

While writing a script for the domain tree tutorial, I felt it was more natural to use (and explain) if the down arrow goes to the first child if there are multiple children (sub)domains.

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This change is 

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (45e7619) 72.32% compared to head (b13872c) 72.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2840   +/-   ##
=======================================
  Coverage   72.32%   72.33%           
=======================================
  Files         257      257           
  Lines        9522     9521    -1     
  Branches     1097     1097           
=======================================
  Hits         6887     6887           
+ Misses       2308     2307    -1     
  Partials      327      327           

Flag	Coverage Δ
backend	82.96% <ø> (+0.02%)	⬆️
frontend	62.58% <100.00%> (-0.01%)	⬇️

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[jmgrady]

Reviewed 1 of 3 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)