Replace Gtk.FileChooserDialog by Gtk.FileChooserNative
While working on #8983 (closed) the problem appeared that flatpak portals cannot be used by Gajim because of the non-native file chooser, see https://github.com/flathub/flathub/pull/362#discussion_r185331109.
Use Gtk.FileChooserNative to improve the situation for users of flatpak Gajim (and to make flathub people happy