Android: Added sf::getNativeActivity() to retrieve a handle to the current activity

This is only implemented for Android builds and requires the user to include the sub-header specifically. Other platforms will trigger a compiler error, if they try to include this header file.

include/SFML/System/NativeActivity.hpp

@@ -0,0 +1,62 @@
+////////////////////////////////////////////////////////////
+//
+// SFML - Simple and Fast Multimedia Library
+// Copyright (C) 2007-2015 Laurent Gomila (laurent@sfml-dev.org)
+//
+// 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_NATIVEACTIVITY_HPP
+#define SFML_NATIVEACTIVITY_HPP
+
+
+////////////////////////////////////////////////////////////
+// Headers
+////////////////////////////////////////////////////////////
+#include <SFML/System/Export.hpp>
+
+
+#if !defined(SFML_SYSTEM_ANDROID)
+#error NativeActivity.hpp: This header is Android only.
+#endif
+
+
+struct ANativeActivity;
+
+namespace sf
+{
+////////////////////////////////////////////////////////////
+/// \ingroup system
+/// \brief Return a pointer to the Android native activity
+///
+/// You shouldn't have to use this function, unless you want
+/// to implement very specific details, that SFML doesn't
+/// support, or to use a workaround for a known issue.
+///
+/// \return Pointer to Android native activity structure
+///
+/// \sfplatform{Android,SFML/System/NativeActivity.hpp}
+///
+////////////////////////////////////////////////////////////
+SFML_SYSTEM_API ANativeActivity* getNativeActivity();
+
+} // namespace sf
+
+
+#endif // SFML_NATIVEACTIVITY_HPP