QGenericUnixServices::openDocument / xdgDesktopPortalOpenFile uses wrong fd mode, breaking snaps

The xdgDesktopPortalOpenFile helper called by QGenericUnixServices::openDocument passes an O_PATH file descriptor to the xdg-desktop-portal D-Bus API:

https://github.com/qt/qtbase/blob/5.13/src/platformsupport/services/genericunix/qgenericunixservices.cpp#L202

This fails for Snap packaged applications, but works for Flatpaks due to some backward compatibility code.

Instead, it should pass an O_RDONLY file descriptor to prove that the application has read access to the file it wants opened (since you can open O_PATH file descriptors for files you can't read), which will work for both Flatpaks and Snaps.  This is what the GLib code calling this interface does:

https://gitlab.gnome.org/GNOME/glib/-/blob/2.64.2/gio/gopenuriportal.c#L107

This problem was originally reported on the Snapcraft forum here:

https://forum.snapcraft.io/t/xdg-open-or-gvfs-open-qdesktopservices-openurl-file-somelocation-file-txt-wont-open-the-file/16824?u=jamesh