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.