Upgrade to Qt 6

[aalex]

Upgrade to Qt 6

[alastaira]

I have made some progress towards this, but there is a considerable amount of effort involved.
In Qt 6 the “surface” API is gone, so the QAbstractVideoSurface subclasses need to be replaced with a QVideoSink, then connect to its videoFrameChanged signal, and pull frames from there.

I have a fork that has followed the Qt5 -> Qt6 upgrade guide but sadly is still not compiling correctly. I think this might be due to other changes in the gstreamer/mingw/msvc dependencies (I'm building on Windows) but don't have time to investigate more at the moment.

[aalex]

In progress! #592