[IMPROVEMENT] Run in background support for Linux

[ekolis]

Please confirm these before moving forward.

• I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
• This improvement refers to an existing feature. If you want to suggest a new feature, please use this template.
• This improvement is not a bug. If you want to report a bug, please use this template.

Describe the improvement

UniGetUI can run in the background (in the system tray) on Windows, but this option doesn't seem to be available on Linux. I'd like this feature to be available in Linux as well.

Describe how this improvement could help users

This would allow updates to download in the background on Linux, and provide easier access to the app.