Add API to disable system proxy per profile

Qt WebEngine should allow an easy configuration for networking proxies, in particular disabling the system proxy.

Currently Qt WebEngine queries QNetworkProxy::applicationProxy() to determine whether a custom proxy should be used. If nothing is set, it will use the system proxy configuration. Unfortunately there is no easy way to configure Qt WebEngine to not use any proxy at all.

One of the problems is that there are various ways in Qt Network to disable the system proxy:

• Configuring with -no-system-proxies
• Calling QNetworkProxy::setApplicationProxy(QNetworkProxy())
• Calling QNetworkProxyFactory::setUseSystemConfiguration(false)
• Calling QNetworkProxyFactory::setApplicationProxyFactory(...)

Not all of these can also be monitored by the API. That is, QNetworkProxy::applicationProxy() will return NoProxy both if an empty proxy has been set, or if the system proxy is not set.

The suggestion therefore is to add

QWebEngineSettings::UseSystemProxies
QQuickWebEngineSettings::UseSystemProxies

to allow explicit control for the WebEngine stack.