Don't overdo it with thumbnailing

lazy_player/main_window.py

@@ -463,7 +463,7 @@ class MainWindow(Gtk.ApplicationWindow, Watcher):
             for i in range(self.list_model.get_n_items()):
                 if item := self.list_model.get_item(i):
                     assert isinstance(item, FileItem)
-                    if not item.thumbnail.value:
+                    if not item.attempted_thumbnail.value:
                         self.thumbnailer.generate_thumbnail(item)
                         break