TH Fixes - Clean-up, efficiency, and Sort Icons fixes

[lrljoe]

Fixes to smooth the Sort Icon Styling approach

• Tidy up main "th" blade - clean up class merging, reduce duplicated code

• TH Bulk Actions - Clean up classes

• TH Collapsing Columns - use Computed Properties, clean up classes

• TH Label (new) - centralise the TH label

• TH Plain - clean up classes

• TH Reorder - clean up classes

• TH Sort Icons - clean up classes, improve performance

• Datatable - clean up attribute assignment

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

1. Does your submission pass tests and did you add any new tests needed for your feature?
2. Did you update all templates (if applicable)?
3. Did you add the relevant documentation (if applicable)?
4. Did you test locally to make sure your feature works as intended?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.53%. Comparing base (eda4c5e) to head (7becf76).

Additional details and impacted files

@@              Coverage Diff               @@
##             development    #2140   +/-   ##
==============================================
  Coverage          90.53%   90.53%           
  Complexity          1980     1980           
==============================================
  Files                205      205           
  Lines               4488     4488           
==============================================
  Hits                4063     4063           
  Misses               425      425           

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