Trying to consolidate hierarchy UI (+ some other fixes)

[igor-dv]

Issue: #151

What I did

Here is the current state of the UI

1. Toggle icon changed to chevron
2. Other icons are removed ( 😢 ). According to @shilman it looks cleaner.

Other changes in the same branch:

1. '/' is set as default to the hierarchySeparator
2. storybook-react-treebeard changed back to react-treebeard

How to test

TBD

[ndelangen]

In the future we will allow addons to customize the treeview/navigation component, I'm OK with this look, it's something we can iterate on after release IMHO.

[codecov]

Codecov Report

Merging #1488 into release/3.2 will increase coverage by 0.16%.
The diff coverage is 30%.

@@               Coverage Diff               @@
##           release/3.2    #1488      +/-   ##
===============================================
+ Coverage        20.27%   20.44%   +0.16%     
===============================================
  Files              240      236       -4     
  Lines             5188     5146      -42     
  Branches           637      622      -15     
===============================================
  Hits              1052     1052              
+ Misses            3641     3624      -17     
+ Partials           495      470      -25

Impacted Files	Coverage Δ
...les/ui/components/left_panel/stories_tree/index.js	100% <ø> (ø)	⬆️
lib/ui/src/modules/api/index.js	0% <ø> (ø)	⬆️
...i/components/left_panel/stories_tree/tree_style.js	25% <ø> (ø)	⬆️
lib/ui/example/client/provider.js	0% <ø> (ø)	⬆️
...ponents/left_panel/stories_tree/tree_decorators.js	31.94% <30%> (-0.92%)	⬇️
app/react-native/src/preview/story_kind.js	0% <0%> (ø)	⬆️
addons/events/src/components/Event.js	0% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
...mponents/left_panel/stories_tree/tree_node_type.js	0% <0%> (ø)	⬆️
... and 24 more

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[shilman]

This is a work of art. Thank you @igor-dv !!! 🎉

[tmeasday]

Looking great!