screenShield: Show detailed notification data if notifications hint it

Allow notifications to set a x-gnome-privacy-scope hint, with values in
['system', 'user']. If all the notifications in a particular source hint
that their privacy scope is ‘system’, don’t hide the notification
details on the lock screen.

This is aimed at fixing the particular case of power notifications: they
contain information which is not private to the user (it relates to the
system: battery state or AC state, which is obvious to anyone who can
see the machine), so hiding the details of a power management
notification when the screen is locked is pointless.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/gnome-shell/issues/726

js/ui/notificationDaemon.js

@@ -352,6 +352,10 @@ var FdoNotificationDaemon = class FdoNotificationDaemon {
         // of the 'transient' hint with hints['transient'] rather than hints.transient
         notification.setTransient(!!hints['transient']);
 
+        let privacyScope = (hints['x-gnome-privacy-scope'] || 'user');
+        notification.setPrivacyScope(privacyScope == 'system' ? MessageTray.PrivacyScope.SYSTEM
+                                                              : MessageTray.PrivacyScope.USER);
+
         let sourceGIcon = source.useNotificationIcon ? gicon : null;
         source.processNotification(notification, sourceGIcon);
     }