add "no songs queued" placeholder

src/ui/play_queue.blp

@@ -1,29 +1,48 @@
 using Gtk 4.0;
+using Adw 1;
+using Gio 2.0;
 
 template $UiPlayQueue: Box {
   name: "play-queue";
 
-  ScrolledWindow {
-    hexpand: true;
-    vscrollbar-policy: always;
-    hscrollbar-policy: never;
+  Stack {
+    visible-child-name: bind $visible_child_name (template.playbin as <$Playbin>.play_queue as <Gio.ListStore>.n-items) as <string>;
 
-    ListView view {
-      show-separators: true;
-      single-click-activate: true;
+    StackPage {
+      name: "empty";
 
-      styles [ "rich-list" ]
-
-      activate => $on_row_activated ();
-
-      model: NoSelection {
-        model: bind template.playbin as <$Playbin>.play_queue;
+      child: Adw.StatusPage {
+        icon-name: "list-remove-all";
+        title: "No songs queued";
       };
+    }
 
-      factory: SignalListItemFactory {
-        setup => $on_song_list_setup ();
-        bind => $on_song_list_bind ();
-        unbind => $on_song_list_unbind ();
+    StackPage {
+      name: "not-empty";
+
+      child: ScrolledWindow {
+        hexpand: true;
+        vscrollbar-policy: always;
+        hscrollbar-policy: never;
+
+        ListView view {
+          show-separators: true;
+          single-click-activate: true;
+
+          styles [ "rich-list" ]
+
+          activate => $on_row_activated ();
+
+          model: NoSelection {
+            model: bind template.playbin as <$Playbin>.play_queue;
+          };
+
+          factory: SignalListItemFactory {
+            setup => $on_song_list_setup ();
+            bind => $on_song_list_bind ();
+            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";
+    }
 }