PageScene: live page rearranging without losing annotations

Creates a single entry ("macro") on the undo stack. Translates a set of items on the page. Input is a list of items; filter out anything that isn't user-placed (which seems harder to do than it should be!)

todo

• code to move objects
  • Proof of concept: press Ctrl+Shift+H to translate roughly 50% of the items 20 pixels leftward.
  • remove the PoC hack
• wire page buttons from !1768 (merged) to behave "live" rather than rebuilding annotator from scratch
  • for rotations
    • DTRT for 180 rotations
    • move annotations for the size changes resulting from 90 degree rotations
  • for deletions
  • for shifts
• undo-able page manipulations: for a future MR? Filed #2510.
• rehide behind experimental mode
• consider more about single-source of truth in PageScene.src_img_data
  • currently when you cancel the annotating, the page rearrangments are persistent within this session
  • Issue #2509 (closed), not specific to this MR (it was like that before).