QNetworkProxy: enable proxy within QtWebKit

[kewde]

We can enable proxy support for QtWebKit (the HTML GUI) through QNetworkProxy very easily.
This would force any traffic coming from the GUI, like external images hosted on website and sent through chat to be routed through the proxy, preventing the leakage of receiver.

proxy.setType(QNetworkProxy::Socks5Proxy);
proxy.setHostName("proxy.example.com");
proxy.setPort(1080);
proxy.setUser("username");
proxy.setPassword("password");
QNetworkProxy::setApplicationProxy(proxy);

In the case of Tor, it can support images hosted on hidden services (.onions).

I would suggest to do this through a Qt dialog so a bug in the Javascript of the GUI is unable to disable the proxy and leak the IP address.