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 

I think `isSomething` method names are usually used for getters, not setters. Also, "not" in method names often makes them harder to understand in the code. So, perhaps `setFileSavedState()` or `setFileDirtyState()`?