This is a union of all possible data types that attributes can have.
Depending on the indicator, the value field can represent a single string,
an array of strings, an integer, an array of integers, a double, an array
of doubles, a selection structure or a notification structure. When
requesting an attribute modification, the Open API program must set
this field to the desired appropriate value(s). When the UIStyler returns
from the attribute query or callback data, it sets this field to proper
value(s) with the indicator field correctly set to indicate the data type
of this field. If this field contains data returned from the UIStyler,
it must be freed by the Open API program when it is no longer needed.
This can be done by calling UF_STYLER_free_value.