This attribute tells strgclass that this gadget is a plain string gadget.
The attribute points to a string which the object copies into the string
gadget's current string value buffer.

When a strgclass gadget's internal stringa_longval or stringa_textval
value changes (usually because the user manipulated the gadget), it sends
itself an om_notify message.  the om_notify message will contain two
attribute/value pairs, GA_ID and either STRINGA_LongVal or STRINGA_TextVal
(depending on what kind of strgclass gadget it is).  Strgclass gadgets
only send a final OM_NOTIFY message (one's with the OPUF_INTERIM flag of
the opupdate.opu_flags field cleared).