Add support for more Network transports (SMB, SFTP)

QNAM supports FTP, HTTP, local files and Qt resources. That's it. (Plus the two internal backends: one for cache-reading and the debug one). It would be nice if we could support more things, like SFTP (transfer protocol over SSH, not FTPS) or SMB on Unix platforms (it's native on Windows).

But, to do that, the first task is to cleanup the QNetworkAccessBackend class, so that it can be made public. Then, once it's public, we can make plugins of those additional protocols, like KDE's KIOSlave system.

Going further, we can even integrate with KIOSlaves and GIO by way of plugins.