ui/pointer: simplify pointer pass through

Get rid of the confused LayerOp and the transparent property from
AreaOp. Add an explicit PassOp to specify whether pointer events
pass-through the current area.

Let AreaOp swallow events even when no handlers are active for the
area. That behaviour is less surprising and allow clients to disable
a widget by keeping its areas but leave out its handlers.

Simplify the pointer.HitResult enum to just a bool: hit or no hit.

Finally, simplify the pointer queue by tracking parent areas and
node with indices.

ui/layout/list.go

@@ -91,7 +91,6 @@ func (l *List) Next() (int, Constraints, bool) {
 	if ok {
 		cs = axisConstraints(l.Axis, Constraint{Max: ui.Inf}, l.crossConstraintChild(l.cs))
 		l.ops.Begin()
-		ui.LayerOp{}.Add(l.ops)
 	}
 	return i, cs, ok
 }