Create a method such as registerOnunload()

[porduna]

Right now, we couldn't find a safe way to run a callback when the user moves from one space to other.

For example, consider a widget showing a resource that requires some exclusive access, and there is a queue that guarantees this exlusivity. Ideally, if the user goes to other space, the widget notifies the server that the user has exited. This way, the server can assume that the session is over and somebody else in the queue can use it.

However, right now there is no method called so the widget can perform this notification. It can rely on polling, but there is no way to support at the same time a more efficient approach. So ideally there would be something like:

gadgets.util.registerOnUnloadHandler(something);

or similar.