Support doing heavy work in the background

[MarcelGarus]

Description

Some operations such as parsing or merging of universes are just intrinsically heavy for big applications. We could move them to the background, allowing us to keep the UI responsive even when lots of parsing is happening.

Hints and Thoughts

Web workers are probably the way to go. They are kind of explicitly created JS threads that don't run on the main event loop (unlike, for example, promises).

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Acceptance Criteria

• The UI should be responsive even during parsing