Add Discord Rich Presence support

Source/Core/UICommon/DiscordPresence.h

@@ -0,0 +1,12 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+namespace Discord
+{
+void Init();
+void UpdateDiscordPresence();
+void Shutdown();
+}  // namespace Discord