Present variables for keywords in stub/heading order.

[johan-ejstrud]

Currently variables under a keyword appear in alfabetic order. This should instead be ordered by order of stub and heading.

Before

STUB="skin_color","sex";
HEADING="eye_color";
VALUES("eye_color")="blue","blue-gray","brown","dark","hazel";
VALUES("sex")="-","female","male";
VALUES("skin_color")="dark","fair","light";

STUB="skin_color","sex";
HEADING="eye_color";
VALUES("skin_color")="dark","fair","light";
VALUES("sex")="-","female","male";
VALUES("eye_color")="blue","blue-gray","brown","dark","hazel";

This also applies for all other keywords that apply for a stub/heading variable.