Allow defining actions in terms of other actions

[StandingPadAnimations]

Right now, every action requires an individual script, which results in actions that are made from “master” scripts. Although this works fine, it can make debugging a nightmare if these scripts don't propagate errors (see #14).

In order to make things easier for developers, a subactions variable will be added as an option for actions, and may look something like this (using translation actions in MCprep as an example):

build_actions:
  translate:
    subactions: 
      - build_pot
    build_pot: 
        script: "dev.py"