Add support for fullscreen on Linux without grabbing the keyboard and mouse (allow alt-tab !)

The fullscreen mode is set using the window manager.

This allows to use alt-tab, alt-F4, volume keys and other even if your application
is running fullscreen

WARNING: To enable this support, you must pass Style::Fullscreen and Style::FullscreenWM
(like Fullscreen Window Manager) to RenderWindow's constructor.

This switch has no effect on other platforms, so you can use it to all your projects.

Note: the fullscreen via Window Manager is not enabled by default (only using Style::Fullscreen)
as potential bugs and performance hit remain to be tested.

![alt_tab](https://f.cloud.github.com/assets/820621/757388/e9c1625c-e698-11e2-953e-80204cbeb3fd.jpg)
![desktop](https://f.cloud.github.com/assets/820621/757389/e9fec3b8-e698-11e2-92fd-c0d424b01a4f.jpg)