[applets] Show KCMs in System Settings

Summary:
BUG: 417836
FIXED-IN: 5.20

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 applets and activate their configure menu items
Observe that the KCMs are opened in System Settings

Reviewers: mart, Plasma

Reviewed By: mart, Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29714