Change ICAddressBook to really use ICContacts

[laugengebaeck]

This will probably be an issue for the next SWT group.

In #396 we added the ICContact class abstracting contacts, but in fact only used it to generate strings to be added to the address book. When we added a sorting functionality in #403, it became apparent that this is not a good idea, as sorting code tends to get really ugly. Nevertheless, we didn't change it since this would have necessitated a big UI change in the ICAddressBookDialog (for details see the linked pull request).

Now, the ICAddressBook should be changed so that it really uses ICContacts. For this, also changes to the sorting code and the UI of the ICAddressBookDialog will be necessary, since the currently used PluggableTreeSpec only supports displaying strings directly.