Changed collapsed icon for CollapsibleWidget

[IgorTatarnikov]

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Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?
The current collapsed icon is currently ▲.

What does this PR do?
Changes the default collapsed icon to ▶ and exposes it as an optional parameter.

How has this PR been tested?

Tested locally.

Is this a breaking change?

No

Does this PR require an update to the documentation?

No

If any features have changed, or have been added. Please explain how the documentation has been updated (and link to the associated PR). See here for details.

Checklist:

• The code has been tested locally
• The code has been formatted with pre-commit

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.53%. Comparing base (3465b1f) to head (ec81b8f).
Report is 85 commits behind head on main.

❗ Current head ec81b8f differs from pull request most recent head a7b7c73

Please upload reports for the commit a7b7c73 to get more accurate results.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #78       +/-   ##
===========================================
+ Coverage   64.04%   90.53%   +26.49%     
===========================================
  Files          33       35        +2     
  Lines         840     1427      +587     
===========================================
+ Hits          538     1292      +754     
+ Misses        302      135      -167     

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