webadmin: detach handlers on hide

- Before this patch the change event handlers were detached
  when the object containing them would be detached from the
  DOM, this could happen many times and it would cause the
  handlers to be detached before we wanted them to. This patch
  detaches the handlers when the popup is hidden instead of
  detached from the DOM.

Change-Id: I56de07f80cbb21fe6362d6df488defe7422cebfb
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1164744
Signed-off-by: Alexander Wels <awels@redhat.com>