[social] 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 are now virtual, and can be reimplemented.
- Moved finishedHandler() slot from all the Facebook*Interface to IdentifiableContentItemInterfacePrivate. This private slot is also virtual and can be reimplemented.
- Other cleanups