Use Background Portal if available for autostart

[hifi]

Refactored to use dbus code generator with the real portal XML spec.

This makes auto-start work "natively" on Wayland desktops like Plasma that provides the portal to non-sandboxed applications as well and avoids us from creating a custom entry.

Testing strategy

TBD

Type of change

• ✅ Refactor (significant modification to existing code)

[hifi]

This didn't really work as-is on either KDE or GNOME. Both created an autostart entry without the Exec key. We need to set the command manually which somewhat defeats the point of using the portal in the first place. The portal spec says you can leave the command out and it'll use the one in the desktop file but that wasn't the case in practice.