Add Linux support

[carmcc]

• [x] Split Windows and Linux files
• [x] Write configuration file
• [x] Start using wg-quick
• [x] Stop using wg-quick
• [ ] Implement "disallowed IPs"
• [ ] Correctly update the VPN status
• [ ] Only request password when necessary (e.g. closing the app always triggers a password prompt)
• [ ] Limit settings (such as autostart and split tunneling)
• [ ] Hide secondary servers (those require OpenVPN, which is not going to be implemented)