Added missing new window icon

There was an image that didn't exist in the develop branch: `icon-new-windows.png`. The idea is showing to the user/admin that the link will be opened in a new windows. It's something like this:

![title icon new window](http://f.cl.ly/items/3g1U3O1o2E343S2U0i1p/Screen%20Shot%202013-07-09%20at%2012.34.16.png)