Allow AREngine to be used independently of ARCodeFile (#2108)

* Make `EmuInstance::cheatFile` use a `unique_ptr`

- Fixes a memory leak, as the cheat file wasn't cleaned up in the destructor

* Split `AREngine` and `ARCodeFile` apart

- Suitable for frontends that have their own way of storing cheats
- Store the cheats in `AREngine` in a `std::vector`
- Apparently cheats are _supposed_ to be executed each frame; I didn't understand this until recently

src/frontend/qt_sdl/EmuInstance.h

@@ -256,7 +256,7 @@ private:
     bool savestateLoaded;
     std::string previousSaveFile;
 
-    melonDS::ARCodeFile* cheatFile;
+    std::unique_ptr<melonDS::ARCodeFile> cheatFile;
     bool cheatsOn;
 
     SDL_AudioDeviceID audioDevice;