messageTray: Don't open the tray from a dwell if we're in a modal grab

If we're in an alt-tab popup or modal dialog, we shouldn't pop up the tray
at all.

https://bugzilla.gnome.org/show_bug.cgi?id=684458

js/ui/messageTray.js

@@ -1561,6 +1561,9 @@ const MessageTray = new Lang.Class({
     },
 
     _trayDwellTimeout: function() {
+        if (Main.modalCount > 0)
+            return false;
+
         // If the user interacted with the focus window since we started the tray
         // dwell (by clicking or typing), don't activate the message tray
         let focusWindow = global.display.focus_window;