Add pre-configured app source lists

Such as:
- Game emulators (https://github.com/omeritzics/Updatium-Emulation-Pack)
- Messaging apps
- Internet browsers
- Productivity apps
- Essential apps
- De-Googling pack
- Icon packs
- Custom FOSS launchers

Todo:
- [ ] Find/make lists to add
- [ ] Implement the built-in list feature in Updatium
- [ ] Make proper built-in list guidelines