Save zoom and magnifier zoom levels when they change

This merge request ensures the zoom and magnifier effects save their initial zoom level even if the user force-reboots or suffers power loss later in the session.

Previously, if you zoom in, log out gracefully, log in, zoom out, force-reboot, then log in again, you would find yourself zoomed in to Plasma when you shouldn't be. This is because the initial zoom level setting was only being saved when either the magnifier effect or zoom effect were destroyed...which most likely won't happen during an unexpected shutdown.

Now, both effects will save the zoom level one second after it has been changed. I do this to avoid hitting the disk too often when spamming the "zoom in" shortcut. The only way you'll lose the setting now, is if kwin somehow dies unexpectedly less than a second after zooming in/out.