Private class cleanup for interfaces

This commit is a cleanup of the private classes for
the different interfaces, 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.
Clean callback functions to private class

This commit provides a cleaner way to handle
callback functions in the private classes.
It introduces virtual handles in
IdentifiableContentItemInterface, that are
used to manage default errors. These handles
are then implemented in Facebook*Interface.
Private class cleanup for social network classes
Minor fixes

The minor fixes includes:
* Adding the explicit keyword to some constructors.
* Setting pointers in private class constructors to 0,
  before initializing them in public constructors.
* Removing useless destructors.
* Factorizing connections. In interfaces and social
  network classes, it is common to connect a QNetworkReply
  to several slots. This commit replaces the numorous
  conenctions by methods that perform the connection, thus
  reducing code duplication.
* Removing (useless) commented code
* Factorizing SocialNetworkInterface constructor. Since the
  constructor of that class performs a lot of actions, and
  that it has 2 different constructors, all the actions are
  now performed by a private function init().