Replace asyncio by Qt Concurrent

Replace all asyncio.Future by the Qt framework QFuture class.

• Code will be more readable and maintainable without all async/await keywords
• Get rid of the need of squamash

See example in https://www.qt.io/blog/asynchronous-apis-in-qt-6

This is not possible with Qt5, as the QFuture binding is missing in PyQt5 and Pyside2:

https://wiki.qt.io/Qt_for_Python_Missing_Bindings

We have to upgrade to Qt6, and switch from PyQt5 to PySide6 which has a QFutureInterfaceBase binding...