Add CheatEngine and support for Gateway cheats (#4406)

* Add CheatEngine; Add support for Gateway cheats; Add Cheat UI

* fix a potential crash on some systems

* fix substr with negative length

* Add Joker to the NonOp comp handling

* Fixup JokerOp

* minor fixup in patchop; add todo for nested loops

* Add comment for PadState member variable in HID

* fix: stol to stoul in parsing cheat file

* fix misplaced parsing of values; fix patchop code

* add missing break

* Make read_func and write_func a template parameter

src/core/cheats/cheats.h

@@ -0,0 +1,37 @@
+// Copyright 2018 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <memory>
+#include <vector>
+#include "common/common_types.h"
+
+namespace Core {
+class System;
+struct TimingEventType;
+} // namespace Core
+
+namespace CoreTiming {
+struct EventType;
+}
+
+namespace Cheats {
+
+class CheatBase;
+
+class CheatEngine {
+public:
+    explicit CheatEngine(Core::System& system);
+    ~CheatEngine();
+    const std::vector<std::unique_ptr<CheatBase>>& GetCheats() const;
+
+private:
+    void LoadCheatFile();
+    void RunCallback(u64 userdata, int cycles_late);
+    std::vector<std::unique_ptr<CheatBase>> cheats_list;
+    Core::TimingEventType* event;
+    Core::System& system;
+};
+} // namespace Cheats