webadmin: Swap KEYPRESS for KEYDOWN in ListModelListBox

There are issues with the KEYPRESS event not being called - swapping
it for KEYDOWN should be good enough a solution in this case.

Change-Id: I006cb4e5e429ada629c23204c72d6cabb6df24bb
Bug-Url: https://bugzilla.redhat.com/987916
Signed-off-by: Lior Vernia <lvernia@redhat.com>