Update for spawn_desktop_exec that calls SpawnTransientUnit

Allows `xdg-desktop-portal` to get app id from PID.

Also updates calls to `cosmic::process::spawn` to spawn in future.

cosmic-app-list/src/wayland_subscription.rs

@@ -107,6 +107,7 @@ pub enum WaylandUpdate {
     Output(OutputUpdate),
     ActivationToken {
         token: Option<String>,
+        app_id: Option<String>,
         exec: String,
         gpu_idx: Option<usize>,
     },