plugins/magnifier: Share zoom factor and initial zoom level settings with Zoom effect

I noticed that, when switching to the Magnifier after using the Zoom effect, my zoom level isn't preserved. I then noticed that the default zoom level for Magnifier is 1.0 (no zoom). This merge request should address both of those issues, since there's no way to set the initial zoom level of Magnifier in the Desktop Effects UI.

Theoretically, this should mean that the Zoom KCM's Zoom Factor setting should apply to the Magnifier as well. This would be useful in this kcm/access merge request where it'd make sense to have a single zoom factor setting. I haven't tested this though.

This change should also mean that zooming in with the Magnifier, then switching to the Zoom effect, should result in you being as zoomed in to the screen as you were when using the Magnifier. That should also work both ways.

I would love to have been able to actually test this, but unfortunately testing the zoom effects just isn't that convenient with my blindness. I could use some help from someone with better eyes? :)