webadmin: Parameterize Event and IEventListener

This to further promote type-safety and not have to unsafely-cast
specific event arguments. Also took the chance to parameterize an
event in ReportsListModel (it wouldn't compile off the bat, and I
preferred to parameterize properly than add a cast).

Change-Id: I1ad96ebbc2a4d2baff514d53dba8a222a08d1306
Signed-off-by: Lior Vernia <lvernia@redhat.com>