Enable QFile supporting asynchronous file access

QFile API documentation explicitly states that it does not support the usual QIODevice signals and its ways of asynchronous access:

https://doc.qt.io/qt-6/qfile.html#signals

We shall investigate and implement those missing signals with VFS in mind. In particular concepts like OnDrive's "file exists use case but is not locally downloaded yet" vs "file has been downloaded" should be investigated on all platforms. How do Unix and Windows systems reflect those states on platform level and make use of it.