Stable API to generate xdg-foreign protocol handle for use in Xdg Desktop Portals.

While under X11 we can use `winId()` to create a window indentifier.

For Wayland, a handle made with the xdg-foreign protocol is needed.
The QPA explicitly use the protocol for the File Chooser portal IIRC.

Qt consumer needs a stable API to get an handle to use portal nicely since the QPA own the exporter.

PS: It is not the same suggestion as QTBUG-73801.