[MlMt] Add “Preview at Left” layout option

[John Cooper]

Quinn Comendant via mailmate wrote (at 7:35 PM on Wednesday, July 9, 
2025):

> It would also be useful to have a fourth option: “Preview Only” 
> that displays *only* the preview window, instead of the composer. When 
> this option is enabled, and Preview is togged on, the composer window 
> will disappear (i.e., hidden below the preview).

I'm personally indifferent to Preview at Left, but I like this idea.
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