Support Qt6

[traversaro]

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Qt5 reached its EOL ~1 year ago, and it will reach its extended EOL in 1 year (see https://endoflife.date/qt). It would be great to also support Qt6.

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[traversaro]

I started some preliminary work on the CMake side in https://github.com/traversaro/yarp/tree/qt6support, now I was blocked as QAbstractVideoSurface used in yarpview is not supported anymore, QVideoSink is the replacement but it is not immediate to port:

• https://forum.qt.io/topic/128382/qt6-porting-guidance-qcamera-and-qabstractvideosurface
• https://doc.qt.io/qt-6/qtmultimedia-changes-qt6.html
• https://stackoverflow.com/questions/71833785/qabstractvideosurface-file-not-found