[cuttlefish] Refactor Screenshot logic
Move all montage/screenshot logic to an unique non-visible item (was `DualMontage`, now `Screenshot`). This frees up `Preview` to just simpler on-screen behavior. It fixes a bug where, after making a non-dual montage, `Kirigami.Theme` bindings within `Preview` would break, stopping to follow the Theme combobox.
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