The integer gadget is really a special case of the string gadget type.
Initialize the gadget as a string gadget, then set the gact_longint flag
in the gadget's activation field.

The user interacts with an integer gadget using exactly the same rules as
for a string gadget, but intuition filters the input, allows the user to
enter only a plus or minus sign and digits.  The integer gadget returns a
signed 32-bit integer in the stringinfo variable longint.

To initialize an integer gadget to a value, preload the input buffer with
an ASCII representation of the initial integer. It is not sufficient to
initialize the gadget by merely setting a value in the longint variable.

Integer gadgets have the longint value updated whenever the ascii contents
of the gadget changes, and again when the gadget is deactivated.