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/stack.go

@@ -55,7 +55,6 @@ func (s *Stack) begin() {
 	}
 	s.begun = true
 	s.ops.Begin()
-	ui.LayerOp{}.Add(s.ops)
 }
 
 func (s *Stack) Rigid() Constraints {