Return app ids with ".desktop" appended to Shell.Introspect

This isn't the correct solution, but it seems to work often enough for window
icons in the screencast dialog.

src/niri.rs

@@ -2184,7 +2184,7 @@ impl State {
             self.niri.dynamic_cast_id_for_portal.get(),
             gnome_shell_introspect::WindowProperties {
                 title: String::from("niri Dynamic Cast Target"),
-                app_id: String::from("rs.bxt.niri"),
+                app_id: String::from("rs.bxt.niri.desktop"),
             },
         );
 
@@ -2193,7 +2193,14 @@ impl State {
             let props = with_toplevel_role(mapped.toplevel(), |role| {
                 gnome_shell_introspect::WindowProperties {
                     title: role.title.clone().unwrap_or_default(),
-                    app_id: role.app_id.clone().unwrap_or_default(),
+                    app_id: role
+                        .app_id
+                        .as_ref()
+                        // We don't do proper .desktop file tracking (it's quite involved), and
+                        // Wayland windows can set any app id they want. However, this seems to
+                        // work well enough in practice.
+                        .map(|app_id| format!("{app_id}.desktop"))
+                        .unwrap_or_default(),
                 }
             });