MudTreeView: Move checkbox icon customization from item to treeview

[henon]

Description

Follow-up to #8661

It just doesn't make sense to be able to set different per-item checkbox icons in a tree view, so I moved the checkbox icon parameters into MudTreeView. Also discovered some minor things we overlooked in #8661

How Has This Been Tested?

existing tests

Type of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation (fix or improvement to the website or code docs)

Checklist

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 90.02%. Comparing base (28bc599) to head (e2c410e).
Report is 82 commits behind head on dev.

Files	Patch %	Lines
...lazor/Components/TreeView/MudTreeViewItem.razor.cs	0.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #8750      +/-   ##
==========================================
+ Coverage   89.82%   90.02%   +0.19%     
==========================================
  Files         412      419       +7     
  Lines       11878    12176     +298     
  Branches     2364     2399      +35     
==========================================
+ Hits        10670    10962     +292     
+ Misses        681      669      -12     
- Partials      527      545      +18     

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[henon]

This copypaste error is responsible for issue #8940