Made a callback for Friend.status change

A convenient callback that can be used by clients to determine when a friend goes online/offline, instead of [frequently querying `m_frienstatus()` and keeping a record of friends' previous statuses](https://github.com/nurupo/ProjectTox-Qt-GUI/blob/9b1bdf61655433760dc02f0258bcf7ed89ea1764/src/core.cpp#L95).