sidebar: Use a proper checked state for the menu button
We don't generally override down for setting the pressed state of menu buttons, instead we use the checked state, which is subtly different visually.
Moreover, using onPressed here causes a weird behaviour where any subsequent press after clicking the "highlight changed settings" menu item will re-open the menu on wayland, probably caused by the same source as https://bugs.kde.org/show_bug.cgi?id=464855 . Using onToggled instead will prevent this.