style: unfill radio buttions in GridPlanWidget

[gselzer]

This PR, closing #339, adds a HACK to prevent the OS accent color from filling up the GridPlanWidget's radio buttons on Windows 11. This HACK seems to be the least-bad of the different options that could resolve the issue, which are:

• Setting the style sheet, as I did here, to override the accent color fill
• Adding the radio buttons to the layout of the QScrollArea itself, which is worse as it alters the hierarchy of the widget.

@tlambert03 and/or @fdrgsp: I'd be happy for your opinions and/or testing. It looks nice to me on Windows 11, and on WSL (Wayland)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.59%. Comparing base (7ec66e5) to head (5b481c6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #344   +/-   ##
=======================================
  Coverage   90.59%   90.59%           
=======================================
  Files          74       74           
  Lines        8379     8380    +1     
=======================================
+ Hits         7591     7592    +1     
  Misses        788      788           

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[gselzer]

Closes #339

[tlambert03]

hilarious 😂 ... works for me! looks exactly the same as before on macos