Add client-side-decorators support trying to follow Gnome3 HIG.

Hi all,
I invite all interested to test these changes.
![pragha-gnome3-hig](https://cloud.githubusercontent.com/assets/733715/5236059/0c72da58-77ff-11e4-9452-5fb111c4cb50.png)

This is major change proposed for the next version.. but "que no cunda el pánico" (Mm. Understandable for Latin Americans.) because it is an optional change.

Then, there are several changes in management of preferences. In short, except for activation of plugins, all changes on preferences dialog should be accepted, and if the dialog is canceled or closed the preferences are restored..

Possible TODO:
 * ~~Check that the desktop support it!! i.ig.: Xfce with compositor disabled fail. Any help here????~~ DONE
 * ~~Detect gnome 3 desktop and activate it by default on first run.~~ DONE
 * Any errors you detect? :wink: 

Regards,
Matias.