Input: Copy current SDL.h/cpp files to impl

This should make reviewing much easier as you can then see what changed
happened between the old file and the new one

src/input_common/sdl/sdl_impl.h

+// Copyright 2017 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include <vector>
+#include "core/frontend/input.h"
+
+union SDL_Event;
+namespace Common {
+class ParamPackage;
+}
+namespace InputCommon {
+namespace Polling {
+class DevicePoller;
+enum class DeviceType;
+} // namespace Polling
+} // namespace InputCommon
+
+namespace InputCommon {
+namespace SDL {
+
+/// Initializes and registers SDL device factories
+void Init();
+
+/// Unresisters SDL device factories and shut them down.
+void Shutdown();
+
+/// Needs to be called before SDL_QuitSubSystem.
+void CloseSDLJoysticks();
+
+/// Handle SDL_Events for joysticks from SDL_PollEvent
+void HandleGameControllerEvent(const SDL_Event& event);
+
+/// A Loop that calls HandleGameControllerEvent until Shutdown is called
+void PollLoop();
+
+/// Creates a ParamPackage from an SDL_Event that can directly be used to create a ButtonDevice
+Common::ParamPackage SDLEventToButtonParamPackage(const SDL_Event& event);
+
+namespace Polling {
+
+/// Get all DevicePoller that use the SDL backend for a specific device type
+void GetPollers(InputCommon::Polling::DeviceType type,
+                std::vector<std::unique_ptr<InputCommon::Polling::DevicePoller>>& pollers);
+
+} // namespace Polling
+} // namespace SDL
+} // namespace InputCommon