ScreenShield: hide Removable devices in the lock screen

Showing the removable devices is potentially a security risk (as
they include network shares). Also, a nautilus launched from there
can't be used, so it's just a way to overload the system.

https://bugzilla.gnome.org/show_bug.cgi?id=681143

js/ui/screenShield.js

@@ -159,8 +159,9 @@ const NotificationsBox = new Lang.Class({
     },
 
     _summaryItemAdded: function(tray, item, dontUpdateVisibility) {
-        // Ignore transient sources
-        if (item.source.isTransient)
+        // Ignore transient sources, or sources explicitly marked not to show
+        // in the lock screen
+        if (item.source.isTransient || !item.source.showInLockScreen)
             return;
 
         let obj = {