Show in tab that a file was not saved yet

If a file is not saved yet, an asterisk is shown before the file name.

This solves #2938 

![screenshot from 2014-09-26 14 05 52 v01](https://cloud.githubusercontent.com/assets/8182792/4428400/f5edf89a-45cb-11e4-9db6-b75a78fb5730.png)
![screenshot from 2014-09-26 14 05 58 v01](https://cloud.githubusercontent.com/assets/8182792/4428403/f8bf1b9e-45cb-11e4-9d08-2e8fa2a7bc60.png)

review @ussuri @devoncarew 

An asterisk is commonly placed after the name, with a separating space in-between. Also, I think all this should be done using the CSS's `::after` pseudo-class, not via direct manipulation of a tab's label.