Add automatic download & setup of Java runtimes

[hedgehog1029]

Progresses #521

Adds support for automatically downloading the correct Mojang-provided Java runtime for modpacks.

Remaining tasks:

• [x] Basic integration
• [ ] Add GUI for managing downloaded runtimes
  • In case there's an issue with the runtime and the user needs to redownload it
• [ ] Allow disabling automatic download in advanced cases
  • For people who prefer to manage their own runtime?
  • Will likely be a hidden setting and not exposed to GUI