Add support for fullscreen on Linux without grabbing the keyboard and mouse.
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.
Small code improvements thanks to MarioLiebisch and Oberon00
Fullscreen via Window manager by default
Fix indentation (replace tabs by spaces)
Merge branch 'master' of github.com:Congelli501/SFML