Fix type conversion for Windows

MSVC uses WString instead of Utf16 so this needs different
conversion in type_conversion.h.

Change-Id: I987acb4ea5ad5a98c94fd992a9de05f602be825d
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>