effects: Better fix for keeping dragged things on top and hiding highlight

Now there is an explicit hierarchy of properties for bubbling up and down the current shared state of "whether there is something being dragged on a scene right now?"

Ideally we could have a special QML type like ButtonGroup which would keep track of a value equivalent to an expression:

any(a.active for a in attachedObjects)

which could then be chained up and the final result passed down. But for (to the best of my knowledge) now no such component exist, so it much be done manually and dirty.