Request: Add job daemon for additional packages

[h8d13]

For slow connections/mirrors this would make loading the Additional Packages section quicker.
Attempted in #4069 with a hacky solution.

This would need a proper framework for jobs/threading. So that when user arrives to this section it is alreayd loaded.

Do note that pacman -Sl is very quick .15s and more optimal way of building the initial repo | name | version stub.

[svartkanin]

With the job running in the background using show should be fine and it is less complex.

This could then also be used to load the mirrorlist