Multiple close/open support

[gogoprog]

Hello,

The following:

myWindow.close();
myWindow.open();

during the same JavaScript frame breaks the behavior and the window can be lost forever.

This PR prevents such issue.

Let me know what do you think

[rlamana]

I understand the issue here, but I'm not sure if this is the right solution. Generally, this only happens when programmatically you close and then open a window, which occurs so fast that the animationEnd event handler of the open operation is executed before the window is even closed. I don't clearly see the use case however, I would approach this by either:

1. ignoring any operation if the previous animation has not ended.
2. enqueuing the operation and execute it whenever the animation has ended by means of promises.