webadmin: confirmation popup - ensure key event bubbling

Catch ONKEYDOWN event on DialogBoxWithKeyHandlers for
invoking relevant command when pressing enter\esc key.
The change is needed for confirmation dialogs that
don't contain widgets that extend FocusWidget
(which adds a DomHandler for KeyPressEvent).

Change-Id: I0f31dea9a5ea502879d450c3ed45aa0d53c4adc2
Signed-off-by: Daniel Erez <derez@redhat.com>