add "no songs queued" placeholder

src/ui/play_queue.blp

@@ -1,9 +1,26 @@
 using Gtk 4.0;
+using Adw 1;
+using Gio 2.0;
 
 template $UiPlayQueue: Box {
   name: "play-queue";
 
-  ScrolledWindow {
+  Stack {
+    visible-child-name: bind $visible_child_name (template.playbin as <$Playbin>.play_queue as <Gio.ListStore>.n-items) as <string>;
+
+    StackPage {
+      name: "empty";
+
+      child: Adw.StatusPage {
+        icon-name: "list-remove-all";
+        title: "No songs queued";
+      };
+    }
+
+    StackPage {
+      name: "not-empty";
+
+      child: ScrolledWindow {
         hexpand: true;
         vscrollbar-policy: always;
         hscrollbar-policy: never;
@@ -26,5 +43,7 @@ template $UiPlayQueue: Box {
             unbind => $on_song_list_unbind ();
           };
         }
+      };
+    }
   }
 }

src/ui/play_queue.vala

@@ -161,4 +161,8 @@ public class Ui.PlayQueue : Gtk.Box {
     [GtkCallback] private void on_row_activated (uint position) {
         playbin.select_track (position);
     }
+
+    [GtkCallback] private string visible_child_name (uint n_items) {
+        return n_items > 0 ? "not-empty" : "empty";
+    }
 }