Port to Qt6

[skadge]

Qt5 is end of life, and it would make sense to port to Qt6 at some point. Nothing urgent (even though Ubuntu for instance plans to drop Qt5 at some point next year), but would be nice.

[JamesMcCrae]

As an update - I have something compiling and running locally (Qt 6.8 on Windows 11 in the below screenshot), but in testing it seems to be buggy in a number of ways (viewport, redraw, mouse events, dock widgets not showing), so it will take some additional work.

The majority of the changes to get it compiling were GL related, switching to classes like QOpenGLWidget and QSurfaceFormat.