Clear notification when the last one is removed

Previously, we switched to not updating the notification popup on a
removal, since this usually respawns a new notification popup
unexpectedly. However, when the last relevant notification is cleared
(ie, by opening/reading the thread before the notification times out and
disappears on its own) we should clear the existing popup if there is
one.

// FREEBIE

js/notifications.js

@@ -15,6 +15,7 @@
     Whisper.Notifications = new (Backbone.Collection.extend({
         initialize: function() {
             this.on('add', this.onAdd);
+            this.on('remove', this.onRemove);
         },
         onclick: function() {
             var last = this.last();
@@ -29,11 +30,6 @@
             this.clear();
         },
         update: function() {
-            if (this.length === 0) {
-                extension.notification.clear();
-                return;
-            }
-
             var setting = storage.get('notification-setting') || 'message';
             if (setting === SETTINGS.OFF) {
                 return;
@@ -124,6 +120,12 @@
             extension.notification.clear();
             this.update();
         },
+        onRemove: function() {
+            if (this.length === 0) {
+                extension.notification.clear();
+                return;
+            }
+        },
         clear: function() {
             this.reset([]);
         }