Fix predicted turn order when units have same CV

Due to JS's lack of a stable sort, when multiple units had the same
counter values, upcoming turns were sometimes displayed out of order. It
gets confusing very quickly when a party member is shown to act next and
the enemy goes instead!
New defaultTargets() method

This centralizes default target determination for skills and items,
which is needed in several places and up until now has been done using
instanceof hacks.  Kind of dumb in a duck-typed environment, actually!
:-)
Implement Dragonflame skill

Not only is this the first multi-target skill, it's also the debut of
Breath damage. Breath attacks are quite unique: They can't miss, and are
the only type of damaging move that uses VIT for damage calculation.
Refactor skill growth

SkillUsable.grow() applies a specified number of experience points to
the skill.  SkillUsable itself refactored to use this internally, and it
can also be called from outside, e.g., for Munch.