[imp] Displaying message when setting articles to Featured or Unfeatured

See https://github.com/joomla/joomla-cms/pull/5511
and
https://github.com/joomla/joomla-cms/pull/5485

This PR implements the display of a message when Featuring or Unfeaturing articles in the Articles and Featured Articles Managers.

One will get something like:
![screen shot 2014-12-25 at 12 34 25](https://cloud.githubusercontent.com/assets/869724/5553865/0548a814-8c33-11e4-9e2f-94019c6dc98d.png)

This has to be tested on latest staging branch