window-list: Fix minimized styling

Commit 039c66e7b7 wrapped the button in a container to
animate transitions, but didn't adjust the `.minimized`
styling to still apply to the button (where it is
expected) rather than the wrapper.

Fix this just like commit c72b8b21 did for the
`.focused` styling.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/342>
(cherry picked from commit 28fd210f28)

extensions/window-list/extension.js

@@ -449,9 +449,9 @@ class WindowButton extends BaseButton {
         super._updateStyle();
 
         if (this.metaWindow.minimized)
-            this.add_style_class_name('minimized');
+            this._button.add_style_class_name('minimized');
         else
-            this.remove_style_class_name('minimized');
+            this._button.remove_style_class_name('minimized');
     }
 
     _windowEnteredOrLeftMonitor(metaDisplay, monitorIndex, metaWindow) {