[KCM/Activities] Use KConfigXT in ui
Summary: Port the ui components to use KConfig XT features to reduce amount of boilerplate code.
Test Plan: kcmshell5 activities
Reviewers: Plasma, ervin, bport
Reviewed By: ervin
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D26398