disable the selection of current mode to fix #2545

This disables the currently selected mode from being "unselected", thus graying out the menu item, see below for an example:

![screen shot 2014-05-27 at 9 03 48 pm](https://cloud.githubusercontent.com/assets/1406149/3099743/faccf35c-e603-11e3-8b15-618e56fb0f65.png)