Support accessing GSettings for Flatpak portal whitelisted keys
Currently when running under Flatpak without the DConf sandbox hole, applications cannot access desktop settings such as those under
org.gnome.desktop.interface via GSettings, even though these have been whitelisted by the Flatpak portal.
This means for example Geary does not respect the desktop font preferences for interface, document and monospaced fonts when running under Flatpak (e.g. geary#989)
A workaround for an application is to determine if it is running under Flatpak or not whenever access to a desktop setting is required, and if so then perform a manual lookup of the setting via libportal/DBus to the portal, and if not use GSettings as usual. This is a terrible development experience however, and abstracting over this kind of platform-specific details is what GLib exists for in the first place. Hence it would be appreciated if GSettings would do just that.