webadmin: Update list box upon arrow key press

ValueChangeEvent is only triggered currently when a selection changes
using the moust or when the user focuses out of the widget. This patch
causes an event to fire when the selection changes using the keyboard.

Change-Id: I4cc9ac7e82c6d1c86886907cecd6eb5b83ac8444
Bug-Url: https://bugzilla.redhat.com/987916
Signed-off-by: Lior Vernia <lvernia@redhat.com>