Moved the change handling from AgentBase to a nested class called Observer.

To allow inheriting both from AgentBase as well as Observer, Observer can't be a QObject thus can't receive the ChangeRecorder's signals itself.
Made the AgentBase private derive from QObject to act as a mediator, i.e. receiving the signals and calling the respective observer method.

Extending AgentBase::init() and ResourceBase::init() to automatically register the observer if the class itself is implementing it.

svn path=/trunk/KDE/kdepimlibs/; revision=789825