frontend: ignore in findbugs several unconfirmed cast errors

Ignore in findbugs errors related to casting EventArgs.
The casting is safe in this case, and a future refactoring
will make findbug warning resolved in any case.(RFE #1034844)
This is a first patch in series, more similar will be submitted.

Change-Id: I2487e6d7d760a68e912caa9c639c70fa7a694631
Signed-off-by: Alissa Bonas <abonas@redhat.com>