Store whether Magnifier is enabled in the config

[SaschaCowley]

Is your feature request related to a problem? Please describe

Enabling NVDA's new magnifier feature does not cause this to be stored across restarts. For users who use the feature frequently, this is likely to be desired.

Describe the solution you'd like

When toggling the magnifier, store whether it is enabled in NVDA's configuration. Add a checkbox to the Magnifier settings dialog to toggle the magnifier on and off.

Describe alternatives you've considered

Implement something like Screen Curtain's temporary mode for the magnifier.

Additional context

[CyrilleB79]

I strongly disagree with introducing another temporary mode. Having both temporary and persistent modes for the same feature is a bad UX. Either the feature should be persistent (as most of NVDA features), or temporary = per-session (as Speech mode).
Actually, I support clarifying Screen Curtain on this matter.