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/ARCodeFile.cpp

@@ -33,17 +33,26 @@ ARCodeFile::ARCodeFile(const std::string& filename)
 {
     Filename = filename;
 
-    Error = false;
-
-    Categories.clear();
-
     if (!Load())
         Error = true;
 }
 
-ARCodeFile::~ARCodeFile()
+std::vector<ARCode> ARCodeFile::GetCodes() const noexcept
 {
-    Categories.clear();
+    if (Error)
+        return {};
+
+    std::vector<ARCode> codes;
+
+    for (const ARCodeCat& cat : Categories)
+    {
+        for (const ARCode& code : cat.Codes)
+        {
+            codes.push_back(code);
+        }
+    }
+
+    return codes;
 }
 
 bool ARCodeFile::Load()