Fix crash on destruction of TelepathyChannelManager

ClientHandler is renamed to ChannelManager to match the QML name, and the Tp::AbstractClientHandler subclass is split into a separate object to avoid pointer management conflicts:

    Tp::ClientRegistrar requires a shared pointer to the
    Tp::AbstractClientHandler subclass being registered; the QML-managed
    ChannelManager object does not mesh well with telepathy's shared
    pointers, which resulted in the object being deleted twice.

    The ClientHandler subclass is now a separate object, so it can be
    managed exclusively by Tp::SharedPtr.