Use methods in KF5-activities to switch to previous/next activity
Since the release of KDE Frameworks 5.68.0 we can now
use the methods exposed in KActivities::Controller to trigger a
switch to the previous or next activity from ShellCorona.
Reviewers: ngraham, ivan, apol
Reviewed By: ngraham, apol
Differential Revision: https://phabricator.kde.org/D28083