[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.