Add ability to add QNAM backends

QNAM already provides the means to add new backends via QNetworkAccessBackend/QNetworkAccessBackendFactory. We should consider making this interface public and adding the means to install such a backend to QNAM.

Note that this change may imply further improvements to the above interface to make it more robust.

We could use QNAME's Ftp backend as proof of concept and provide a separate way of shipping (removed from QNetwork).