Add power sabotage ability to spy

This adds a power sabotage ability to the ra and ts spies in an effort to make them more useful.
I haven't managed to find any suitable sounds I can mix, so there isn't much feedback to the player being infiltrated apart from a yellow selection bar and the disabled overlay for power plants.
I don't think PowerOutageModifier is a good class name, but I haven't been able to come up with something better.
This still doesn't quite feel right to me... A couple of distinct ideas that might improve this:
* Move the PowerModifier stuff into the Power trait, possibly with a flag.  Rename this to `PowerOutagebar`.
* Rename this to `AffectedByPowerOutage` to make it clearer that this has nothing to do with triggering outages.