Add Discord RPC (#161)

* Add discord-rpc submodule

* Add Discord RPC

* Fix up Discord status

* Fix CMake because MacOS sucks

* Slightly less hacky fix

include/discord_rpc.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#ifdef PANDA3DS_ENABLE_DISCORD_RPC
+#include <discord_rpc.h>
+
+#include <cstdint>
+#include <string>
+
+namespace Discord {
+	enum class RPCStatus { Idling, Playing };
+
+	class RPC {
+		std::uint64_t startTimestamp;
+		bool enabled = false;
+
+	  public:
+		void init();
+		void update(RPCStatus status, const std::string& title);
+		void stop();
+	};
+}  // namespace Discord
+
+#endif