Remove power_mod; Allow items to replace powers on the action bar

Closes #1130

The power_mod feature gave us the ability to mutate powers by combining them. However, how they were mutated wasn't obvious, and it only worked with weapons.

This new implementation will cause some data duplication in flare-game (unique powers for slingshots and bows), although I think the end result is clearer. "x replaces y" is much simpler and flexible than "x modifies y".

I'll post a PR on the flare-game repo that brings it up to speed with these changes.