app: Implement basic uninstallation
This is merely the basic implementation on uninstall operation. It prominently lacks:
- App specific handling of configuration and other setup removal
- App specific handling of packages not be removed
- Purging of packages instead of removing them (some FreedomBox installed files will get removed sometimes).
I believe we can merge this because this shows a 'experimental' warning to users and we can continue to work on improvements.