Added sf::setPowersavingEnabled()

This provides a convenient way to temporarily keep the system from going into sleep mode or showing the screensaver/turning off the display, as described in issue #772.

include/SFML/System.hpp

@@ -35,6 +35,7 @@
 #include <SFML/System/InputStream.hpp>
 #include <SFML/System/Lock.hpp>
 #include <SFML/System/Mutex.hpp>
+#include <SFML/System/Power.hpp>
 #include <SFML/System/Sleep.hpp>
 #include <SFML/System/String.hpp>
 #include <SFML/System/Thread.hpp>

include/SFML/System/Power.hpp

@@ -0,0 +1,58 @@
+////////////////////////////////////////////////////////////
+//
+// SFML - Simple and Fast Multimedia Library
+// Copyright (C) 2007-2014 Laurent Gomila (laurent.gom@gmail.com)
+//
+// This software is provided 'as-is', without any express or implied warranty.
+// In no event will the authors be held liable for any damages arising from the use of this software.
+//
+// Permission is granted to anyone to use this software for any purpose,
+// including commercial applications, and to alter it and redistribute it freely,
+// subject to the following restrictions:
+//
+// 1. The origin of this software must not be misrepresented;
+//    you must not claim that you wrote the original software.
+//    If you use this software in a product, an acknowledgment
+//    in the product documentation would be appreciated but is not required.
+//
+// 2. Altered source versions must be plainly marked as such,
+//    and must not be misrepresented as being the original software.
+//
+// 3. This notice may not be removed or altered from any source distribution.
+//
+////////////////////////////////////////////////////////////
+
+#ifndef SFML_POWER_HPP
+#define SFML_POWER_HPP
+
+////////////////////////////////////////////////////////////
+// Headers
+////////////////////////////////////////////////////////////
+#include <SFML/System/Export.hpp>
+#include <SFML/System/Power.hpp>
+
+
+namespace sf
+{
+////////////////////////////////////////////////////////////
+/// \ingroup system
+/// \brief Allows or disallows the system to assume being idle
+///
+/// Depending on the local machine's settings, the system
+/// might disable the screen or go into sleep if no input is
+/// registered for an extended period of time.
+///
+/// This functions allows suspending these features while
+/// the current application is still active.
+///
+/// \param duration Time to sleep
+///
+/// \return Whether the request has been successful
+///
+////////////////////////////////////////////////////////////
+bool SFML_SYSTEM_API setPowersavingEnabled(bool enable);
+
+} // namespace sf
+
+
+#endif // SFML_POWER_HPP