Fix icons on MacOS

[soininen]

Re spine-tools/Spine-Toolbox#1265

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes in Toolbox repo have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.09%. Comparing base (59671d6) to head (6f7e7f7).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #247   +/-   ##
=======================================
  Coverage   63.08%   63.09%           
=======================================
  Files         168      168           
  Lines       17977    17979    +2     
  Branches     2070     2070           
=======================================
+ Hits        11341    11343    +2     
  Misses       6030     6030           
  Partials      606      606           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.