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.

src/SFML/System/Android/NativeActivity.cpp

@@ -0,0 +1,39 @@
+////////////////////////////////////////////////////////////
+//
+// 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.
+//
+////////////////////////////////////////////////////////////
+
+////////////////////////////////////////////////////////////
+// Headers
+////////////////////////////////////////////////////////////
+#include <SFML/System/NativeActivity.hpp>
+#include <SFML/System/Android/Activity.hpp>
+
+namespace sf
+{
+////////////////////////////////////////////////////////////
+ANativeActivity* getNativeActivity()
+{
+    return priv::getActivity()->activity;
+}
+
+} // namespace sf

src/SFML/System/CMakeLists.txt

@@ -14,6 +14,7 @@ set(SRC
     ${INCROOT}/Lock.hpp
     ${SRCROOT}/Mutex.cpp
     ${INCROOT}/Mutex.hpp
+    ${INCROOT}/NativeActivity.hpp
     ${INCROOT}/NonCopyable.hpp
     ${SRCROOT}/Sleep.cpp
     ${INCROOT}/Sleep.hpp
@@ -75,6 +76,7 @@ else()
         set(PLATFORM_SRC ${PLATFORM_SRC}
             ${SRCROOT}/Android/Activity.hpp
             ${SRCROOT}/Android/Activity.cpp
+            ${SRCROOT}/Android/NativeActivity.cpp
             ${SRCROOT}/Android/ResourceStream.cpp
             ${SRCROOT}/Android/ResourceStream.cpp
         )