messageTray: Re-add the hotcorner when the panel is updated

The activities button may come and go at any moment now that we
have a dynamic panel. We need to re-check the activities button
whenever the panel is updated.

https://bugzilla.gnome.org/show_bug.cgi?id=694038

js/ui/main.js

@@ -131,6 +131,9 @@ function start() {
         screenShield = new ScreenShield.ScreenShield();
     else
         screenShield = new ScreenShield.ScreenShieldFallback();
+
+    // The message tray relies on being constructed
+    // after the panel.
     panel = new Panel.Panel();
     messageTray = new MessageTray.MessageTray();
     keyboard = new Keyboard.Keyboard();