LibWeb: Move ImageDecoder client connection singleton to its own file

This will allow us to use it in more places around LibWeb.

Userland/Libraries/LibWeb/ImageDecoding.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2020-2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibImageDecoderClient/Client.h>
+
+namespace Web {
+
+ImageDecoderClient::Client& image_decoder_client();
+
+}