KServicePrivate is already a friend to be able to access convertToQVariant

KServiceAction needs to do the same

- use the strings in a QStringList directly
- slightly better readability: less if-else-nesting, return early

GIT_SILENT

... also when built without -DKCONFIG_USE_DBUS

Fixes: https://bugs.kde.org/457285

GIT_SILENT

We've had multiple people accidentally building KConfig without QML support and then complain that something down the line is breaking

To make that harder add an explict option to disable the QML stuff

Containers (both Qt and STL) don't have virtual destructors, which could
be an issue when trying to delete an object of a derived class via a
pointer to the base class.