Support XDG Autostart Specification

[Biont]

The Desktop Application Autostart Specification describes a procedure to handle applications that are supposed to start automatically after a desktop session has initialized.

This is widely used, and supported by many programs that have have builtin configuration options to toggle autostart (for example Nextcloud or Steam), this might be useful for some.

My hope is that much of the existing code can be reused to implement this. The launcher could then handle autostarts with a simple sway-launcher-desktop.sh autostart somewhere in the user's config