- allow negative values in step field (ORM measurement dialog)
- fix AttributeError if using model without undo stack
- fix ValueError for "Add" button in match dialog
- separate "Write strengths" menu item further from "Read strengths"
- add hotkey for "Read strengths"
- find models in breadth-first manner, not depth first
- fix bug that caused incorrect update when changing plot, especially from
  plots with many curves to plots with less curves