Added contexts to various Unknown strings

Fixes bug https://bugzilla.gnome.org/show_bug.cgi?id=659968

js/ui/contactDisplay.js

@@ -63,7 +63,7 @@ const Contact = new Lang.Class({
                         this.individual.full_name ||
                         this.individual.nickname  ||
                         email                     ||
-                        _("Unknown");
+                        C_("contact", "Unknown");
         let aliasLabel = new St.Label({ text: aliasText,
                                         style_class: 'contact-details-alias' });
         details.add(aliasLabel, { x_fill: true,

js/ui/notificationDaemon.js

@@ -483,7 +483,7 @@ const NotificationDaemon = new Lang.Class({
     },
 
     _onTrayIconAdded: function(o, icon) {
-        let source = this._getSource(icon.title || icon.wm_class || _("Unknown"), icon.pid, null, null, icon);
+        let source = this._getSource(icon.title || icon.wm_class || C_("program", "Unknown"), icon.pid, null, null, icon);
     },
 
     _onTrayIconRemoved: function(o, icon) {

js/ui/status/power.js

@@ -212,7 +212,7 @@ const DeviceItem = new Lang.Class({
         case UPDeviceType.COMPUTER:
             return _("Computer");
         default:
-            return _("Unknown");
+            return C_("device", "Unknown");
         }
     }
 });