Adding Change Variant and Change Animation skill components & hooks

This adds new hooks and components that will allow users to create skills that can change a unit's variant or animation. Edits have also been made to the promotion code to force it to use a unit's default class and variant, so no weird stuff happens there and players can recognize what they're promoting from/to. If you think that's unnecessary, feel free to remove/edit.

As always, make any changes you want, or reject the merge request if it's bad/you don't want it in the engine. I can always make a writeup and keep it as a mod.