Window Strip is only for Plasma Active, so we don't want to see
it in the UI for Window Switcher Layouts as it would not properly
work on the desktop anyway.
Accomplished by a simple property definition on the service file.
A new service type is introduced which is used by the KCM to find
all available window switcher layouts. This makes it finally possible
to have 3rd party window switchers.
Also the tabbox finds the packaged QML file through the service
definitions.
Desktop switcher is not yet in packaged version (UI to configure
is missing).
REVIEW: 103951
