title: Group several nodes for action and be notificated after completion
body: <p>I have a SKNode with some children. For these children I'd like to apply actions which should start a the same time (sample code, k1000ms is a constant): </p>
  
  <pre><code>let children = self.children        
  for var i=0; i&lt;children.count; i++ {
    children[i].runAction(SKAction.moveTo(CGPoint(x: 0.0, y: 0.), duration: k1000ms))
  }
  </code></pre>
  
  <p>The point I don't know how long an iteration takes so starting actions for a huge number of children might result in a delay between the first and last started action.
  The solution is obviously apply the actions to all nodes first and then starting the actions with a single call (hopefully the OS takes care of proper synchronization but this is an other issue).</p>
  
  <p>Question 1: How to apply the actions to all nodes first and afterwards start all at once?</p>
  
  <p>Question 2: How to get notified all actions are performed?</p>