ui/layout: don't stretch Align child

Let the caller decide whether the constraints should be stretched.

Also unexport Constraint (not Constraints) methods, they weren't
pulling their weight.

Finally, don't force cross axis constraint minimum to 0 in List.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/layout/list.go

@@ -217,11 +217,8 @@ func (l *List) Layout() Dimens {
 
 func (l *List) crossConstraintChild(cs Constraints) Constraint {
 	c := axisCrossConstraint(l.Axis, cs)
-	switch l.CrossAxisAlignment {
-	case Stretch:
+	if l.CrossAxisAlignment == Stretch {
 		c.Min = c.Max
-	default:
-		c.Min = 0
 	}
 	return c
 }