Implement NetworkManager

For now there are two methods:

- `NETWORK:IsUrlAllowed()`: Check whether access to a certain URL is allowed.
- `NETWORK:HttpRequest()`: Perform an HTTP request.

By default access to the network is disabled for all target hosts. It
can be enabled by setting `HttpEnable=1` in the preferences. Individual
hosts have to be added to `HttpAllowHosts` as a comma separated list to
allow access.

See included docs for more details on usage.

extern/IXWebSocket-11.3.2/ixwebsocket/IXWebSocketErrorInfo.h

@@ -0,0 +1,22 @@
+/*
+ *  IXWebSocketErrorInfo.h
+ *  Author: Benjamin Sergeant
+ *  Copyright (c) 2017-2018 Machine Zone, Inc. All rights reserved.
+ */
+
+#pragma once
+
+#include <cstdint>
+#include <string>
+
+namespace ix
+{
+    struct WebSocketErrorInfo
+    {
+        uint32_t retries = 0;
+        double wait_time = 0;
+        int http_status = 0;
+        std::string reason;
+        bool decompressionError = false;
+    };
+} // namespace ix