[Tablet] Reconsider design of Note Editor Previewer

• Ref: https://github.com/ankidroid/Anki-Android/pull/18724
• Ref: https://github.com/ankidroid/Anki-Android/pull/19988

2.24 introduced a split pane preview to the note editor. It also moved from layout-xlarge to layout-sw600dp.

A number of users have requested that a toggle is added: https://forums.ankiweb.net/t/uncomfortable-using-new-split-screen-in-editor/69656 (comments split across Discord, Play Store and Reddit).

By far the most convincing argument is by phannan:

I appreciate all the hard work that went into doing the tabs, and unfortunately, it isn’t helpful for me. Especially in the edit screen, the old version works a lot better for me. I am lower vision, so everything is blown up. Losing a part of the screen makes both sides unusable. (See attached) Yes, I can flip to portrait mode, but then I lose half the card top to bottom. I’d appreciate having the option to turn this off, especially in the edit screen.

-- https://forums.ankiweb.net/t/uncomfortable-using-new-split-screen-in-editor/69656/3?u=david

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I would propose tackling this in 2 phases:

2.24.X

• Evaluate and reconsider sw600dp with real world examples
  • Very likely: move from sw (smallest side width) to w (current width), and select a larger value
  • Determine a justified value for foldables
  • Potentially revert to sw720dp (roughly layout-xlarge)

2.25.0 (expedited release for this change)

• Add a setting to Note Editor toggling the sidebar (and enabling the preview icon)