Tab should have a way to cancel close

There should be an API for canceling the attempt to close a tab, similar to the way we can cancel closing of a Window. Without this, there is no way of, for example, prompting a user if they're sure they want to close the tab, giving them a final chance to veto the pending change.

In Window, there is an onCloseRequest event, which has the following description: "Called when there is an external request to close this Window. The installed event handler can prevent window closing by consuming the received event."