[applets] Show KCMs in System Settings or Info Center, as appropriate

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 applets and activate their configure menu items
Open applets and click on their settings buttons
Observe that the KCMs are opened in System Settings or Info Center
(depending on the context)

Reviewers: mart, #plasma

Reviewed By: mart, #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29712