Better handle game environment reloads with a progress bar of some sort

[Cervator]

At present there is a warning on the advanced game setup screen that if you continue down the advanced path there may be a substantial wait in trying to load the next screen. We should really be able to do better and show a progress bar and not simply lock the screen to where it seems like the game has frozen.

Actual handling of the game / module environment is somewhat tricky, as @TheFlash98 learned during his GSOC project on inching toward full support for multi-world. Several pesky bugs were squashed and a secondary sub-environment of sorts is now used as part of the create game process. Maybe we can still improve it and keep the UI thread separate from whatever gets locked up while doing an environment reload?

For some related issues see the multi-world project board at https://github.com/orgs/MovingBlocks/projects/15 or issues #2774, #1743, or probably others hiding. This is a tricky area, here be monsters!