WebUI: fix incorrect behavior of ESC button on combobox
    
    When combobox is opened then ESC key should close it. There was a bug
    that ESC key closed also the dialog. It was caused by bad keyboard event
    handling. The CB was closed by keydown event and the dialog by keyup.
    
    Therefore the propagating of keyup and keydown event is stopped when CB
    is opened (when the event is fired on CB element).
    
    https://fedorahosted.org/freeipa/ticket/6388
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>