Cleanup in social

This cleanup includes
* Qt pimpl with a clean inheritance of private classes
* Reply "handlers" are moved to private classes, and are called
  using Q_PRIVATE_SLOT macro. Each Facebook*InterfacePrivate
  implements it's own handlers, and the parent class,
  IdentifiableContentItemInterfacePrivate provides default
  handlers for error.
* Small fixes leading to QtCreator marking some lines as error
  (Q_INVOKABLE should be before static).
* Connecting QNetworkReply to slots often required 3 calls to
  connect, now they are factorized, and
  IdentifiableContentItemInterfacePrivate provides a method
  to perform these connections, instead of many duplicated
  code.
* Other minor cleanups (removed useless calls to disconnect
  before deleting a QNetworkReply)