Cleanups

- Use Qt pimpl macros Q_D, Q_Q, Q_DECLARE_PRIVATE to have clean private classes with inheritance for ContentItemInterface and all subclasses.
- Moved slots like defaultErrorHandler to IdentifiableContentItemInterfacePrivate, declared as private slots using Q_PRIVATE_SLOT.
- The private slots now calls protected methods in IdentifiableContentItemInterface, that have a default behaviour and can reimplemented.
- Moved finishedHandler() slot from all the Facebook*Interface to IdentifiableContentItemInterfacePrivate. This private slot calls an empty method in IdentifiableContentItemInterface that can be implemented in subclasses.