[GUI] revert contact -> buddy wording

PART 2

Change-Id: I2115d9db0b0441dfebf0649f102d011fedcf9cb2
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1188
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>