[IMPROVEMENT] Separate simultaneous operations from downloads

### Please confirm these before moving forward.

- [x] I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
- [x] This improvement refers to an existing feature. If you want to suggest a new feature, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=new-feature&projects=&template=feature-request.yml&title=%5BFEATURE+REQUEST%5D+%28Enter+your+description+here%29).
- [x] This improvement is not a bug. If you want to report a bug, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=bug&projects=&template=bug-issue.yml&title=%5BBUG%5D+%28Enter+your+description+here%29).

### Describe the improvement

Right now you can run multiple installers at a time, that's the only way to have multiple downloads at a time but often multiple installers shouldn't run as the same time.

Would it be possible to separate the options to have multiple downloads and single installs at a time or even intelligently sort the installs by download size so the smallest package installs first while the biggest package installs last so it has the most time to download in the background.

### Describe how this improvement could help users

Faster, wouldn't be waiting for the next download to start until the first install is complete. Multiple installs can fail, only 1 installer can use the windows installer service at a time so then you are juggling retries or waiting until one is complete anyways.

No file or test is named in the issue. Start by locating the code that schedules downloads and installers, then review how concurrent operations are limited. Done means downloads can proceed concurrently while installer concurrency is independently limited, avoiding conflicts with the Windows Installer service.

Written by the indexing model from the issue text.