[GUI] revert contact -> buddy wording

PART 7

Change-Id: Ie82d0de35ea2c6916f2d964d75e497bbc21e49f4
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1193
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>