Send desktop notification when Element Desktop is started hidden and there are unread messages

### Description

I use `element-desktop --hidden` to start Element in background when I open my session on my computer. However, I can't see whether I have unread messages this way ; I have to unhide Element. I have enabled mail notification to workaround this.

### Steps to reproduce

- Close all your Element sessions and apps
- Use another account to send a message to yourself (or ask a friend)
- Run `element-desktop --hidden`
- See if you get a notification

### Version information

- **Platform**: desktop
- **OS**: Ubuntu 20.04
- **Version**: 1.7.9