file: Unfold scanned booklets into sequential pages

[sjg20]

Unfold scanned booklets into sequential pages

When a saddle-stitched booklet is taken apart (e.g. by pulling the
staples) and fed through a scanner, each scanned page holds two booklet
pages side by side, and the sheets only come out in reading order on the
outermost sheet. This adds an Unfold booklet operation that recovers
normal sequential pages automatically.

What it does

• Splits each scanned page down the vertical middle into a left and a
  right half (no trimming — the full halves are kept, no page dropped).
• Reorders the halves into reading order using the standard saddle-stitch
  imposition. For an 8-page booklet (4 scanned pages) the scans hold
  [8|1] [2|7] [6|3] [4|5] and unfold to 1 2 3 4 5 6 7 8.
• Creates a new max stack and supports undo, mirroring the existing
  duplicate operation.
• Available from the Duplicate… context submenu (and as an action).

Structure

• File::bookletOrder() — pure helper computing the reading order;
  covered by a unit test (testBookletOrder).
• File::addImageAsPage() — the page-writing logic factored out of
  copyTo() so the new path reuses it (no behaviour change to copyTo).
• File::unfoldBooklet() → Desktopmodel::opUnfoldBooklets() /
  unfoldBooklet() → UCUnfoldBooklet undo → desktop action/slot/menu.

Tests

• testBookletOrder — verifies the imposition for a 4-sheet booklet and
  that every source page contributes exactly one left and one right half.
• testUnfoldBooklet — unfolds the test stack, checks the new stack is
  named testfile_unfold, is a Max, has twice the page count, and that
  undo removes it.

Note: the full GUI build could not be exercised locally (the sandbox
lacks the QtStateMachine dev headers used by desktopwidget.cpp); the
core objects and the test were compiled individually and the full
build/tests rely on CI.