Description of problem:
OnClose event starts several times after close SplashScreen. The event also starts when it is switched to other perspective.

Version-Release number of selected component (if applicable):
6.1.0.ER5

How reproducible:
-

Steps to Reproduce:
1. Create SplashScreen plugin. (Extensions -> Plugin Management -> New... -> New SplashScreen). Add some alert to onClose event. For example:

function() {
    alert("Closing!!!");
}

2. Refresh browser.
3. Open perspective where new SplashScreen plugin should be placed.
4. Close SplashScreen. "Closing" alert is displayed.
5. Navigate to other perspective. *)
6. Open perspective where new SplashScreen plugin should be placed.
7. Close SplashScreen. **)

Actual results:
*) "Closing" alert is displayed again.
**) "Closing" alert is displayed twice.

Expected results:
*) "Closing" alert is not displayed.
**) "Closing" alert is displayed once.

Additional info:
If you repeat procedure: switch to other perspective, return back and close splashscreen, alert is displayed increasingly.