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/test/compatibility/python/websockets/echo_server.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python
+
+# WS server example
+
+import asyncio
+import os
+import websockets
+
+
+async def echo(websocket, path):
+    while True:
+        msg = await websocket.recv()
+        # print(f'Received {len(msg)} bytes')
+        await websocket.send(msg)
+
+host = os.getenv('BIND_HOST', 'localhost')
+print(f'Serving on {host}:8766')
+
+start_server = websockets.serve(echo, host, 8766, max_size=2 ** 30)
+
+asyncio.get_event_loop().run_until_complete(start_server)
+asyncio.get_event_loop().run_forever()