Add the "confirmclose" event

[pvgoran]

I needed the ability to prevent the popup from being closed under certain conditions, for example, to allow the user to double-check and either confirm or cancel close. (This ability is present in https://github.com/jquerytools/jquerytools which I try to migrate from.) So I added the confirmclose event that is called before any changes are made, if the event returns non-true, then close is cancelled.