[applet] Show KCM in System Settings

Summary:
CCBUG: 417836

Depends on D29711

Test Plan:
Right-click on applet and activate configure menu items
Observe that the KCM is opened in System Settings

Reviewers: mart, Plasma

Reviewed By: mart, Plasma

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D29715