(js) Add Mailbox.prototype.$selectedMessageIndex

UI/WebServerResources/js/Mailer/Mailbox.service.js

@@ -267,6 +267,16 @@
     return _.find(this.$messages, function(message) { return message.uid == _this.selectedMessage; });
   };
 
+  /**
+   * @function $selectedMessageIndex
+   * @memberof Mailbox.prototype
+   * @desc Return the index of the currently visible message.
+   * @returns a number or undefined if no message is selected
+   */
+  Mailbox.prototype.$selectedMessageIndex = function() {
+    return this.uidsMap[this.selectedMessage];
+  };
+
   /**
    * @function hasSelectedMessage
    * @memberof Mailbox.prototype