alias generator desktop files

this allows UIs to know that this is really okular and may show okular
instead (considering the generators are nodisplay=true)

specifically this allows the xdg-desktop-portal-kde to show okular in
its appchooser dialog even though the actual mimetype match happend
against (e.g.) okularApplication_pdf.desktop and is NoDisplay

(mobile curious has inconsistent naming, the main desktop file is
definitely org.kde.okular.kirigami.desktop)