[INTERNAL] added IChatDisplay interface

Added an interface to access different chat display implementations in
the same manner.

Change-Id: I91f6aa6cff6455321fa067dde62705941c92075b
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1908
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>