Change default style for qt6

[gdementen]

Anaconda recently packaged (py)qt6 and made it the default qt version. It mostly works but the default style on Windows 11 is a mess.

It is generally ugly (e.g. too much space in menus), SpinBoxes are buggy (e.g. no visible buttons), the table headers buttons are almost invisible, etc.

Changing the default style to 'windowsvista' helps though. It's still not super nice but not as bad as the Windows11 style.

Reference: https://stackabuse.com/styling-pyqt6-applications-default-and-custom-qss-stylesheets/

[gdementen]

@alixdamman : CCing you because you'll probably have (or already had) the same problem for iode and I am curious to know how you solved that

[alixdamman]

I am curious to know how you solved that

I didn't solved anything (yet). FWIW, I am using PySide6 and the Qt Designer but I don't think it solves anything.
I guess the current Qt-based iode interface is also ugly but my first priority right now is to make it work.

If you are curious to see the look of the iode interface, you can play with the beta version by installing it:

pip install --index-url https://test.pypi.org/simple/ --no-deps iode
pip install --index-url https://test.pypi.org/simple/ --no-deps iode-gui