[INTERNAL] cleaned up SkypeStylChatDisplay

Made fields and methods private and / or final. Removed useless this
pointers and removed the silentClear method from this class and the
ChatControl class as it is was never used.

Change-Id: I6ed409d2ac76990684217e3eb2a1bac9da8af6c4
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1907
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>