New TIdIMAP4.SearchMailBox() options
skHeader and skKeyword were previously declared in the TIdIMAP4SearchKey enum, but were ignored by the (UID)SearchMailBox() methods. They have now been implemented. In the case of skHeader, a new FieldName member has been added to TIdIMAP4SearchRec to specify the desired header name. Use the existing Text member to specify the desired header value.