FileChooserWidget: Don't show recent items with private hint set

GtkRecentInfo includes an is_private flag that is supposed to indicate whether the recent entry should be shown outside of the application(s) that actually added it.

This change makes the file chooser's recent list obey that hint, so that an application won't offer to open recent entries that it probably can't open. The hint isn't applied to the recent folder list, because it makes some sense that even if the recent file isn't openable, the user might want to work on a file in the same folder.