refactor!: remove multi-world support

[skaldarnar]

Contains

Remove code and abstractions related to mutli-word support during the "Advanced" game creation flow.

Context

The multi-world feature was added as a GSOC project back in 2018 by @TheFlash98. For more information, checkout the original forum post GSOC 2018 - Multiple Worlds or the project page.

While the world generation flow now supports to "create" multiple worlds and configure them separately, these multiple worlds were never experianceable in-game.

How to test

Create a new game via the "Advanced" game setup flow.

Outstanding before merging

• [X] disallow adding more than one world
• [X] replace world lists with single WorldSetupWrappers in screen classes
• [x] remove dropdown related code in universe setup screen
• [x] remove remaining dropdown related code
• [x] remove selected world label, deduce selected world from world generator selection
• [x] don't include "Height Map" world generator as possible selection

Follow-ups

• merge screens (except for world config screen)
• merge world config screen in

[jdrueckert]

@skaldarnar refactoring steps so far remove the option to add more than one world and removes respective code, in particular the former world lists are now only single WorldSetupWrappers... I think a good next step would be to adjust the screens by replacing the world dropdowns by simple fields. That should allow us to get rid of a bunch of ui-related code.