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.cpp

@@ -0,0 +1,51 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#include <chrono>
+#include <string>
+#include <discord_rpc.h>
+#include "citra_qt/discord_impl.h"
+#include "citra_qt/ui_settings.h"
+#include "common/common_types.h"
+#include "core/core.h"
+
+namespace DiscordRPC {
+
+DiscordImpl::DiscordImpl() {
+    DiscordEventHandlers handlers{};
+
+    // The number is the client ID for Citra, it's used for images and the
+    // application name
+    Discord_Initialize("461729900748079114", &handlers, 1, nullptr);
+}
+
+DiscordImpl::~DiscordImpl() {
+    Discord_ClearPresence();
+    Discord_Shutdown();
+}
+
+void DiscordImpl::Pause() {
+    Discord_ClearPresence();
+}
+
+void DiscordImpl::Update() {
+    s64 start_time = std::chrono::duration_cast<std::chrono::seconds>(
+                         std::chrono::system_clock::now().time_since_epoch())
+                         .count();
+    std::string title;
+    if (Core::System::GetInstance().IsPoweredOn())
+        Core::System::GetInstance().GetAppLoader().ReadTitle(title);
+    DiscordRichPresence presence{};
+    presence.largeImageKey = "citra_logo";
+    presence.largeImageText = "Citra is an emulator for the Nintendo 3DS";
+    if (Core::System::GetInstance().IsPoweredOn()) {
+        presence.state = title.c_str();
+        presence.details = "Currently in game";
+    } else {
+        presence.details = "Not in game";
+    }
+    presence.startTimestamp = start_time;
+    Discord_UpdatePresence(&presence);
+}
+} // namespace DiscordRPC