Fixed DisplayName conversion to vcard.

Based on VCARD spec "FN" field is only one string, the old implementation store a string list
and does not change the property type, that causes the following error:

Variant value is a QStringList but the property's value type is neither CompoundType or ListType

Changing the property to store a single string fixes the problem.

Change-Id: I7a7857fb2c498518d7940410d0bb65558e230fec
Reviewed-by: Matthew Vogt <matthew.vogt@qinetic.com.au>
Reviewed-by: Christopher Adams <chris.adams@jollamobile.com>