Make sure KDE Okular can open pdf files in webdav directories

For webdav directories mounted by Dolphin, the flatpak version Okular cannot open it correctly but Brave can.

To fix this problem, we can use this workaround.

It is a bit tricky when using Fedora Atomic. Here is the proper way of doing it.

1. Go to /var/lib/flatpak/app/org.kde.okular/current/active/export/share/applications/ and copy org.kde.okular.desktop to /home/darius/.local/share/applications
2. Edit org.kde.okular.desktop, revise Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=okular --file-forwarding org.kde.okular @@u %U @@ to Exec=/usr/bin/flatpak run --branch=stable --arch=x86_64 --command=okular --file-forwarding org.kde.okular @@u %F @@. Note the %F
3. Save the file. Revise the pdf opening property, point the desktop file to the revised one.
4. Then it is done.