Use GtkFileChooserNative for bookmark and recent files integration
Pika Backup already needs to depend on a lot of filesystem stuff (e.g. network, full home access, gvfsd), so it wouldn't earn much on the sandboxing side. But the native file chooser is nicer because it keeps file chooser settings (e.g. sort order, hidden files toggle) across all apps, it shows the host bookmarks and the recent files are shared too across apps and Nautilus, all this through the flatpak sandbox.