What has to be done
We need to migrate our GUI code from Qt5 to Qt6.

Subtasks:

• Some of the API will have changed, so some reworking of the code will be necessary.
• Probably need to get rid of qnanopainter. It does not support Qt6 and they don't seem to have plans for it in the future. Furthermore, Qt6 supports OpenGL accelerated rendering out of the box, so this library should not be needed.

How do you know it has to be done
Qt5 will not be officially supported past May 2023, which is coming up quite soon. We should get ahead of this.

Starting points
Just do it!

Don't forget (can delete these when done)

• Put the issue in a milestone
• Assign it to one of the projects
• Add appropriate labels
• Mention dependent/blocked issues if any