Add flagNode() method

Split out node flagging into its own method.
Add touch support

Add a boardTap() method that fires onTouchStart.

This allows (more or less) for long-clicking on mobile devices, to
substitute for right-clicking.
Fix occasional mis-clicks from touch events

Cancel the touch timout (this.touched) whenever onTouchEnd or
onTouchMove fires, so that no stale taps get fired.