CamelObject: Implement the notify() method.

Just a placeholder so subclasses can safely chain up, since GObjectClass
does not implement its own notify() method.
CamelOfflineStore: Chain up in the notify() method.

CamelObject provides a safety net for this method now.
CamelIMAPXStore: Chain up in the notify() method.

CamelObject provides a safety net for this method now.