Initial work on callback functions cleanup

This commit introduces an inheritance for the
(default)XxxHandler methods, that were used
to react to a QNetworkReply. It helps
reducing code complexity, and makes the
object hiearchy even cleaner.
Cleanup in the slot system and connections

This commit provides a cleaner slot system. Signals are connected
to slots in private classes, and those slots calls a protected
method in the publi class (that can be reimplemented if needed).

IdentifiableContentItemInterfacePrivate also provide a method
to perform the usual connections at the same time, removing the
need of writing 3 lines of connections all the time.