GameSignals: update member variables correctly when the game is destroyed
RackWidget: fix a bug when the rack is filtered (wrong variable used)
Board: allow one more type of invalid word when previewing a played word
TimerWidget: new "chrono" mode, triggered with Ctrl+click
TimerModel: add a utility method
HintsDialog: display the cost of each hint (optional)
Topping: remove the ScoreWidget and add a TimerWidget