D22370: Simplify previous-/nextActivity code in kactivities-cli

[Christian Muehlhaeuser]

muesli added a reviewer: ivan.

REPOSITORY
  R6 KActivities

REVISION DETAIL
  https://phabricator.kde.org/D22370

To: muesli, ivan
Cc: kde-frameworks-devel, LeGast00n, michaelh, ngraham, bruns