Soulshift ability implementation

happened to notice that `SoulshiftAbility` overrides `trigger()` to set targets. This might not be _wrong_ but it's a weird way to do it as `trigger()` is overridden very infrequently. Normally the logic would be in `checkTrigger()` or in a TargetAdjuster.

Might be some obscure bugs with adjusting the number of triggers (that event is checked between `checkTrigger()` and `trigger()`, or it might not matter.

Also, the ability is supposed to be optional but I don't see where in the code it is set optional. Need to test.