Fixes #15347

Summary of the issue:
#15347 raises 2 issues with handling the results of downloading add-ons when the add-on list has been refreshed by switching to a different add-on list view tab.
This is due to the download task relying on an add-on being visible in the add-on list.

Description of user facing changes
Installing add-ons and then switching tabs should not result in any errors and downloads should be successful

Description of development approach
Up the max workers for downloading to 10, meaning up to 10 add-ons can be downloaded simultaneously
Fix handling of completed downloads to be safer, use the list view model for downloading rather than just the add-on model so that the context of the add-on list view is not needed
Fix up various misleading typing annotations
Add an "UNKNOWN" category for add-on status rather than just causing an error, this will allow add-ons with an unknown status to appear in the incompatible add-ons tab instead of just being hidden