Enable the platform plugin for QtNetwork

Reuse the loader code from the QtGui platform plugin.

List of possible uses, from old internal task QT-406:

• the default Certificate Authority (CA) list
• the SSL ciphers allowed (e.g., disabling of low-grade ciphers)
• system proxy settings (i.e., user configuration)
• ability to intercept some networking signals, like sslErrors() and authenticationRequired(): again, user configuration, like "don't tell me
  that this certificate has expired" or KWallet integration
• backend for the Bearer Management system integration (for example, NetworkManager requires D-Bus)
• Default QNetworkAccessManager HTTP user-agent.