Private class cleanup

This commit is a cleanup of the private classes, and
provides an hiearchy of the private classes that
matches the public ones.
Fix errors in QtCreator and added forgotten entries

This commit fixes a compilation warning, because of some
Q_PROPERTIES macros did not have a READ entry. It also
fixes all the warnings and errors that were displayed in QtCreator.
It also adds the Facebook entries that were not exported to the plugin.
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.
Cleaned the private class for SocialNetworkInterface
Added explicit keywords when needed
Set pointers to 0 in private classes constructors
Moved the protected slots system to private classes

This commit moves all the callback functions, that
are connected to the network replies back to the
private implementation.
Removed useless destructors
Simplified connections in FacebookInterface

This commit introduces a private function that
performs connections, instead of performing the
three classical connections all the time.
Removed useless commented code and factor SNI constructor