Add Discord Rich Presence Support (#3883)

* Initial Discord RPC support

Build with Discord Presence ON

Fix RPC detection

Fix Time elapsed on pause; will now continue to count.

* Fix CI builds with compile flag

Addressed reviews

Fix silly mistakes

Fix 'Not in-game' display

class instead of namespace

Fix

Revamped

remove redundant code

Using Pimpl pattern

* Implement Null class

* Fix config updation

* Addressed All Reviews

* externals/discord-rpc : Updated to latest commit

src/citra_qt/discord_impl.h

@@ -0,0 +1,20 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include "citra_qt/discord.h"
+
+namespace DiscordRPC {
+
+class DiscordImpl : public DiscordInterface {
+public:
+    DiscordImpl();
+    ~DiscordImpl();
+
+    void Pause() override;
+    void Update() override;
+};
+
+} // namespace DiscordRPC