screenShield: defer deactivation until all messages are shown

Right now when a user types their password to unlock their session
we end up getting an unlock signal from GDM right away.  We then
proceed to deactivate the screensaver before the user has a chance
to read his messages.

This commit makes sure we clear out the message queue before processing
the deactivation request.

https://bugzilla.gnome.org/show_bug.cgi?id=704347

js/ui/screenShield.js

@@ -1121,6 +1121,12 @@ const ScreenShield = new Lang.Class({
     },
 
     deactivate: function(animate) {
+        this._dialog.finish(Lang.bind(this, function() {
+            this._finishDeactivate(animate);
+        }));
+    },
+
+    _finishDeactivate: function(animate) {
         this._hideLockScreen(animate, 0);
 
         if (this._hasLockScreen)