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.h

@@ -0,0 +1,25 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+namespace DiscordRPC {
+
+class DiscordInterface {
+public:
+    virtual ~DiscordInterface() = default;
+
+    virtual void Pause() = 0;
+    virtual void Update() = 0;
+};
+
+class NullImpl : public DiscordInterface {
+public:
+    ~NullImpl() = default;
+
+    void Pause() override {}
+    void Update() override {}
+};
+
+} // namespace DiscordRPC