Don't focus to the tab which is going to be removed.

This fixes #569. In the previous revision, onTabClose method tried to focus to the first child even if it was going to be closed as a member of closing tree. Then TabSelect event was unexpectedly fired, TabWindow.prototype.handleEven (defined in resource:///modules/WindowsPreviewPerTab.jsm) raised an error, and blocked all following event handlers for TabSelect event. As the result, cached tabs list were not cleared and Ctrl-Tab feature was killed.