add a new enum type
idx - serial number of the new enum.
If another enum with the same serial number
exists, then all enums with serial
numbers >= the specified idx get their
serial numbers incremented (in other words,
the new enum is put in the middle of the list
of enums).
If idx >= get_enum_qty() or idx == -1
then the new enum is created at the end of
the list of enums.
name - name of the enum.
flag - flags for representation of numeric constants
in the definition of enum.
returns: id of new enum or -1.
long add_enum(long idx, string name, long flag);