desktoppackage: Better integration with KConfigPropertyMap

In Plasma 6 configuration page emits tons of error about missing cfg_* properties during page loading:

ConfigurationAppletPage.qml:58: Error: Cannot assign to non-existent property "cfg_enabledCalendarPlugins"

While this is probably caused by the new stricter rules of Loader item initialization, it still has something do with the way we construct initial properties map: the least we can do is omit dummy system properties coming from Qt runtime, such as "objectName" or "__42".

Fortunately, Plasmoid.configuration is just a KConfigPropertyMap, so it has convenient keys() method which we absolutely should use.