Add multi-monitor support

[hertg]

On a multi-monitor setup, the login prompt is displayed on every monitor.
The non-primary monitors should just display the background image.

[DevBefell]

This is how I temporally fixed this issue, since I am using nody-greeter (it has multi monitor support), in public I created a empy body secondary.html and a index.yml accordingly to nody-greeter dracula theme, this just gives me a blank/black screen on my secondary monitor.