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/flex.go

@@ -77,7 +77,6 @@ func (f *Flex) begin() {
 	}
 	f.begun = true
 	f.ops.Begin()
-	ui.LayerOp{}.Add(f.ops)
 }
 
 func (f *Flex) Rigid() Constraints {