make sure host okular shows up

when using (e.g.) dolphin as a flatpak it runs stuff through the app chooser dialog but that dialog is incapable of handling okular correctly. okular is a bit of an odd ball because it installs a separate desktop file for every file format it supports (thereby technically allowing default handling). unfortunately since currently portals always prompt which app to use (until a clear winner choice has been made) this system falls flat on the face because now we somehow need to get okular into our preference list but the actual preference we get passed in from the upstream portal is 'okularApplication_pdf' not 'org.kde.okular'. Very unfortunate set of circumstances that. To mitigate this problem we manually map the backend files to the main okular file and thereby get okular to show up in the choice dialog.

this is not a problem when okular is installed as a flatpak because there the mimetype list of the flatpak's desktop file is comprehensive