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 

Sorry, I can see now that it's not. However, if the approach stays the same (), then `isFileNotSaved` should change to `isFileNotSaved(bool saved, [EditorTab tab])`, and the body of the `forEach` should reuse it.