Fix updating the system address book without reloading the whole thing.
Also introduce a memory optimisation. It is a bit of a hack but can be disabled. The loading of the System Address Book is dependent on the application name. KMail, Kontact and KAddressbook are allowed to load the SAB by default. Other apps that use the standard address book will not load the SAB or the resource's cache.

This reduces load times and memory footprint. If the SAB is needed the app name can be added to the list in the resource config, in key "SystemAddressBookWhiteList".