menu button: Revisit style class one more time
menu button: Revisit style class one more time

Use .popup, which is already in use elsewhere, fits better with
the other button differentiating style classes, and does not
interfere with menus.

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita