Loaders: Add cancelation to all file loaders.

Mainly, for HTTP which might be stalled trying to connect (especially if
you're not near your PC right now and it's in your recent, for example.)

Core/FileLoaders/RetryingFileLoader.cpp

@@ -72,3 +72,7 @@ size_t RetryingFileLoader::ReadAt(s64 absolutePos, size_t bytes, void *data, Fla
 
 	return readSize;
 }
+
+void RetryingFileLoader::Cancel() {
+	backend_->Cancel();
+}