data/kde-portals.conf: Fallback to GTK portals for Settings

Add a fallback to the GTK portal for Settings. This makes sure that GTK
apps always get valid fonts and related settings.

See: https://bugzilla.redhat.com/show_bug.cgi?id=2240211
See: https://pagure.io/fedora-kde/SIG/issue/396
See: https://github.com/flatpak/xdg-desktop-portal/issues/1017
See: https://github.com/flatpak/xdg-desktop-portal/issues/1112
See: https://bugs.kde.org/show_bug.cgi?id=474746

BUG: 474746

See: !231
(cherry picked from commit 93f96d31)