[versit] SubTypeLandline does not imply TEL;TYPE=ISDN

The Landline sub type of QContactPhoneNumber should not be exported
as ISDN, as this is a more specific annotation. Instead, export
Landline simply as TYPE=VOICE.

A new sub type value is defined for importing TEL records that
claim ISDN type.

Change-Id: Id5afc25597527f4e36dbb23897dce508876642a0
Reviewed-by: Renato Araujo Oliveira Filho <renato.filho@canonical.com>
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>