Distribution List handling

In order to use the existing distribution lists for the CardDAV interface, some
changes have been implemented regarding the distribution list handling. This
document describes those changes.

Terms and Definitions

(a) Contact
A contact is a container object for addressbook-related properties. A contact is
uniquely identified by a combination of its context-, folder- and object-ID.

(b) Distribution List
A distribution list is a collection of address entries (members). Each address
entry represents either another Contact, or only an e-mail address and
optionally a name, a so called One-Off address entry.

(c) One-Off Address Entry
A One-Off address entry is an independant address, i.e. an address aentry that
does not represent another existing contact.

(d) Distribution List Member
A distribution list member is an address entry of a distribution list.

Usage of folder IDs

Until now, only the context- and object-IDs were used by distribution lists to
identify a referenced contact. From now on, also the folder-ID is used. This is
necessary to support different contact storages in the future. Therefore, the
new field folder_id (type String) is introduced for the Distribution