Move antialiasing, hinting, and rgba-order settings from gnome-settings-daemon xsettings plugin to gsettings-desktop-schemas
Moving this from https://github.com/flatpak/flatpak/issues/2861. The problem is that, when running flatpak apps in Wayland-based desktops other than GNOME, gnome-settings-daemon is usually not installed, but this is the only place other than xsettings where the settings are stored. Therefore, the settings are not exported by xdg-desktop-portal's settings portal, and GTK winds up falling back to non-antialiased text.
Discussing with Carlos, we agreed that moving the schemas to gsettings-desktop-schemas makes sense. Not only is it generally a better place, but also it's required by GTK, so it's guaranteed to be installed. We need to update gnome-settings-daemon, gsettings-desktop-schemas, and GTK all in tandem. Carlos says GTK can do g_settings_schema_source_lookup() to see if the new settings exist in gsettings-desktop-schemas, and fall back to gnome-settings-daemon if not, so.
Cost: the tiny minority of users who change font settings will lose their customizations and need to configure them again, but this will be a one-time issue and I doubt many users will even notice. The benefits of providing antialiasing to non-GNOME desktops should exceed the cost.