multi-level "sel back/forward"

[pett]

Implement multi-level selection-only undo/redo as "sel back/forward" and attach to left/right arrow keys.

Once implemented, add an "Undo" button to zone-selection dialog, probably disabling it if unrelated select changes occur afterward.

[pett]

Setting a selection is complex and multi-level undo is rarely actually needed, The generic undo/redo mechanism seems to handle this need well enough.