The core library is what firewall backends will use to inherit
and get powers to control the system. The inheritance should
use IFirewallClientBackend, that needs to be registered via a
FirewallClient, which in turn is what's going to be used in
Qml.

All calls to FirewallClient will just be forwarded to the internal
IFirewallClientBackend, with the exception of signals: the inner
class should trigger the parent's signals as it's what's connected
in the Qml.

Log represents a line of log from the firewall
Rule represents a firewal rule (incoming or outcoming)

Most of those are abstracted enough for backend implementations.