Rework/retune puzzle loading. Save generated puzzle. Add scene margin.

Used to have a scene rect change per piece: now there should be one per
scene. Loading seems to go much faster, but there is still work to do
in co-ordinating preview loading, running the loading widget, loading
pieces, positioning pieces and bringing up the final view cleanly.

The generated puzzle is saved so that the user never gets a re-shuffle
if he/she has not started the puzzle yet. The scene margin keeps the
ConstraintVisualizer (ie. the resize handles) clear of the pieces.