TODO

Missing features:
  Left click for live traps
  Other notations
  Copy notation
  Move annotations
  Board orientation cue
  Clocks for finished games
  Double-headed arrows
  Premoves
  Chat

Infelicities:
  Check move legality
  Check win conditions
  Better layout
  Optimise match, tree
  Exception handling
  Make move illegal but still draw it when arrowset modified
  Logging
  Harlog the shown position (after current move)
  Grabbing arrows from empty squares doesn't work for inferred arrows
  Move recalculated when tree changes, unnecessarily
  Change error calls to something else
  Cycling through branches loses current position; similarly for pathPos & arrows
  What is the point of the Update type ?
  fromServer / getUpdates mess

Bugs:
  Kill threads on game end
  Key selector: exclude modifiers