dialog system: add a side-by-side alignment feature.

This will let us put two controls side by side (e.g. in disjoint
columns of a multi-col layout) and indicate that instead of the
default behaviour of aligning their top edges, their centreline (or,
even better if available, font baseline) should be aligned.

NFC: nothing uses this yet.

dialog.c

@@ -221,6 +221,7 @@ static union control *ctrl_new(struct controlset *s, int type,
     c->generic.handler = handler;
     c->generic.context = context;
     c->generic.label = NULL;
+    c->generic.align_next_to = NULL;
     return c;
 }