New domain preference: SOGoIMAPCASServiceName

Set this to the service name expected by the CAS server if it differs
from SOGoIMAPServer. This is useful to request a CAS ticket for service
imap://imap.domain.com while connecting through imapproxy on imap://127.0.0.1:1143

SOGoDAVAuthenticator is not updated since it doesn't really use the imap code.

SoObjects/SOGo/SOGoDomainDefaults.h

@@ -43,6 +43,7 @@
 - (NSString *) mailDomain;
 - (NSString *) imapServer;
 - (NSString *) sieveServer;
+- (NSString *) imapCASServiceName;
 - (NSString *) imapAclStyle;
 - (NSString *) imapAclGroupIdPrefix;
 - (NSString *) imapFolderSeparator;