window-list: Use more appropriate fallback icon

'icon-missing' is not an actual icon name. It somewhat works
because an invalid icon name will fallback to the correct
'image-missing', however for apps the generic app icon is
a better fallback.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/315>

extensions/window-list/extension.js

@@ -165,7 +165,7 @@ class WindowTitle extends St.BoxLayout {
             this._icon.child = app.create_icon_texture(ICON_TEXTURE_SIZE);
         } else {
             this._icon.child = new St.Icon({
-                icon_name: 'icon-missing',
+                icon_name: 'application-x-executable',
                 icon_size: ICON_TEXTURE_SIZE,
             });
         }