Fix portal urls cache on X11
When I was reading QPA code at first, I was thinking that windows and cocoa QPAs have a QMimeData for all the run time, while xcb and wayland create them on every owner change. It seems I've misread the xcb code as the portal cache bugs on it now.
This changes the cache to use the last transfer ID that should make it unaware of the implementation detail while still allowing the consumer to get portal urls until another file is copied.
Edited by Ilya Fedin