Fix nested Flexible Layout direction problem

[charlesh88]

Closes #7635

• More specific approach to CSS class application for column vs. row layouts.
• Added layout direction CSS classing to c-fl-container__frames-holder.
• Switched toolbar icon and titling for better parity with 'toggle' approach used elsewhere.

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Author Checklist

• Changes address original issue?
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Reviewer Checklist

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[ozyx]

Looking good so far, have a few comments and suggestions

[charlesh88]

All change requests addressed, changes pushed including an update to latest master.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 56.40%. Comparing base (5a06b51) to head (8e823ab).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7637      +/-   ##
==========================================
- Coverage   56.68%   56.40%   -0.29%     
==========================================
  Files         672      672              
  Lines       27183    27182       -1     
  Branches     2634     2635       +1     
==========================================
- Hits        15409    15331      -78     
- Misses      11447    11521      +74     
- Partials      327      330       +3     

Flag	Coverage Δ
e2e-full	23.55% <ø> (-17.98%)	⬇️
e2e-stable	59.99% <ø> (+0.05%)	⬆️
unit	49.20% <0.00%> (-0.03%)	⬇️

Files	Coverage Δ
src/plugins/flexibleLayout/toolbarProvider.js	88.46% <ø> (ø)
...s/flexibleLayout/components/ContainerComponent.vue	10.63% <0.00%> (-0.24%)	⬇️
...ugins/flexibleLayout/components/FlexibleLayout.vue	8.05% <0.00%> (+0.10%)	⬆️

... and 11 files with indirect coverage changes

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[ozyx]

Good stuff! Couple things remaining:

• need to fix one failing test which looks to be due to changes in flexible layout behavior
• this PR needs a quick regression test