[REFACTOR] extracted Viewer filter

The filter for filtering out contacts of the contact list during a
running Saros session might be tricky for new developers. Moved the
filter to a separate class so it should be easier to find out why
contacts may vanished from the contact list during a running session.

Change-Id: I0813686257fa3e0e4d2b561242dd12db8801e852
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1181
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>