[applet] Show KCM in System Settings

Summary:
CCBUG: 417836

Depends on D29711

As this will depend on Frameworks 5.71, it will be landed on master after branching,
ending up in Plasma 5.20.

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

Reviewers: jgrulich, mart, Plasma

Reviewed By: jgrulich, mart, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29716