[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