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/IXSelectInterrupt.cpp

@@ -0,0 +1,48 @@
+/*
+ *  IXSelectInterrupt.cpp
+ *  Author: Benjamin Sergeant
+ *  Copyright (c) 2019 Machine Zone, Inc. All rights reserved.
+ */
+
+#include "IXSelectInterrupt.h"
+
+namespace ix
+{
+    const uint64_t SelectInterrupt::kSendRequest = 1;
+    const uint64_t SelectInterrupt::kCloseRequest = 2;
+
+    SelectInterrupt::SelectInterrupt()
+    {
+        ;
+    }
+
+    SelectInterrupt::~SelectInterrupt()
+    {
+        ;
+    }
+
+    bool SelectInterrupt::init(std::string& /*errorMsg*/)
+    {
+        return true;
+    }
+
+    bool SelectInterrupt::notify(uint64_t /*value*/)
+    {
+        return true;
+    }
+
+    uint64_t SelectInterrupt::read()
+    {
+        return 0;
+    }
+
+    bool SelectInterrupt::clear()
+    {
+        return true;
+    }
+
+    int SelectInterrupt::getFd() const
+    {
+        return -1;
+    }
+} // namespace ix