window-list: Fix active state

Commit c72b8b21 fixed the styling of the active window's button,
but missed that the `active` property uses the style information
as well.

Adjust it to use the correct actor when checking for the style class.

Closes https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/529

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/342>
(cherry picked from commit a5a92026ac)

extensions/window-list/extension.js

@@ -223,7 +223,7 @@ class BaseButton extends DashItemContainer {
     }
 
     get active() {
-        return this.has_style_class_name('focused');
+        return this._button.has_style_class_name('focused');
     }
 
     // eslint-disable-next-line camelcase