feat(wayland): Window::SetAppId

xdg_toplevel.set_app_id, so the compositor can match the window to its
.desktop file: the icon, the task switcher, activation.

interfaces/Crafter.Graphics-Window.cppm

@@ -157,6 +157,9 @@ export namespace Crafter {
         void StartUpdate();
         void StopUpdate();
         void SetTitle(const std::string_view title);
+        // The xdg app id, which the compositor matches to a .desktop file
+        // for the icon, the task switcher and activation.
+        void SetAppId(const std::string_view id);
         // Tells the compositor's input method that a text field has focus
         // (an on-screen keyboard opens) and where its caret is, in physical
         // pixels, or that none has. Remembered: it is applied again whenever