resources: Remove SetWatching/IsWatching code from Watch

This removes some boilerplate from the Watch methods which can be baked
into the engine instead.

This code should be checked for races and locks to make sure we only
start resources when it makes sense to.

pgraph/actions.go

@@ -289,6 +289,9 @@ func (g *Graph) Worker(v *Vertex) error {
 	// the Watch() function about which graph it is
 	// running on, which isolates things nicely...
 	obj := v.Res
+	// TODO: is there a better system for the `Watching` flag?
+	obj.SetWatching(true)
+	defer obj.SetWatching(false)
 	processChan := make(chan event.Event)
 	go func() {
 		running := false