app, event_loop: first-frame-committed hook and foreign_toplevel app_id only

Two independent changes, both blockers for a working desktop session through the loginmanager-daemon handoff and the dock's running-app icons.
`App::on_first_frame_committed` is a new trait hook fired exactly once, immediately after the very first `wl_surface.commit` of a rendered buffer on the main surface. `AppData` grows a `first_frame_committed: bool`, `draw_frame` now returns whether this call performed that first commit, and `try_run` invokes the hook after the borrows held during the draw are released. Used by the loginmanager-daemon-aware crustace path to signal "ready to be presented" back to the daemon as soon as the GPU has the first frame — the actual present can still be deferred under VT switching (no DRM master yet), but the client-side commit is the right edge for handoff.
`toplevel_display_id` in the foreign-toplevel-list handler no longer falls back to `info.title` or `info.identifier` when `info.app_id` is empty. Smithay creates each `ext-foreign-toplevel-list-v1` handle with `app_id = ""` and `init_new_instance` flushes a `done` immediately, so subscribers used to see the protocol-level identifier (a 32-char `Alphanumeric` random token) as the "app id" of every new toplevel — and remained stuck on it whenever the client's real `set_app_id` arrived between done events but the subscribing app's matcher couldn't resolve it to a `.desktop` entry. Returning the raw `app_id` (empty or not) makes that first transient `done` ignorable by the consumer's own empty-string guard; the second `done`, carrying the real app id, is processed normally.

src/app.rs

@@ -293,6 +293,14 @@ pub trait App: 'static
 	/// Return any pending messages from external sources (timers, async, etc.).
 	fn poll_external( &mut self ) -> Vec<Self::Message> { vec![] }
 
+	/// Called once, immediately after the first buffer for the main surface
+	/// has been rendered and committed to the compositor. Note: this is the
+	/// client-side commit, not the compositor's present — under VT switching
+	/// the actual present can be deferred (no DRM master yet), so a handoff
+	/// signal that fires here is enough to tell a parent "ready to be
+	/// presented as soon as my VT becomes active".
+	fn on_first_frame_committed( &mut self ) {}
+
 	/// Called when the surface is asked to close (compositor request, titlebar button,
 	/// layer-shell closed event). Return `true` to allow the application to exit,
 	/// or `false` to cancel.