Now that the handling for multiple address books is in place, this
commit adds initial support for creation and deletion of address books.
These operations obviously require support from the backend, so the
interface gains two new methods. All properties of the address book
passed to `CreateAddressBook()` may be unset (e.g. when a client sends a
MKCOL request without a body), except for the path, which is always set.
It is up to the backend to put any desired default values in place.
This is the equivalent of #127 (and #140) for CardDAV and finally allows
backends to serve different address books to different users.
While I'm breaking the interface, correct one last instance of
"Addressbook" to "AddressBook" (in `AddressBookHomeSetPath`).
As the implementation evolves, it will be necessary to have more tests
to assert we don't break anything when making changes. This commit
introduces a test setup to test that server and client can handle the
address book discovery with various parameters. The test setup should be
easily extendable to cover even more ground as needed.
