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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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