Refine tree view a11y

[fatadel]

The issue fixes #5674.

The original issue mentions inaccessibility of the .treeViewAppendageColumn element (the filename). However, during development I've also noticed inaccessibility of the label frames. As discussed in the matrix channel, for now I just increased their opacity level. Shall we create an issue for further improvement of the dimming functionality? @canova

Appendage Column

Before

After

Label Frames

Before

After

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.71%. Comparing base (ac4bb14) to head (554c509).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5779   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files         317      317           
  Lines       31216    31216           
  Branches     8499     8595   +96     
=======================================
  Hits        26756    26756           
  Misses       4030     4030           
  Partials      430      430           

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