Moving GUI out of the controllers

[mrvisscher]

Big refactor that moves GUI interaction out of the controllers and into specialized ABAction's. This introduces a new design philosophy in which widgets are only responsible for their own state, controllers are only responsible for Brightway's state, and actions are responsible for the logic for moving between states. This:

• Separates the UI from the controllers, making them initializable before UI init, preventing circular imports
• Moves specialized user interaction from widgets, clearing up where user interaction happens
• Makes user interactions easily testable by unit-testing ABActions (>90% coverage right now)
• ABActions inherit QActions, so they can easily be added to buttons, or (context)menus.

ABAction flowchart

Checklist

• Update the documentation, please follow the numpy style guide.
• Update tests.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, feature, ui, change, documentation, breaking, ci
  as they show up in the changelog.
• Add a milestone to the PR for the intended release.
• Request a review from another developer.

[marc-vdm]

Also some documentation TODO:

• Make contributing.md a smaller file with references the below:
• Add files explaining important items (file for actions, controllers, signals etc)
  • Why do they exist?
  • When (not) to use them?
  • Relationships between other things
• File on how to update tests

[marc-vdm]

remove?

[marc-vdm]

remove?