menus: make sure menubar style doesn't override real menuitems
menus: make sure menubar style doesn't override real menuitems

Since we now in GTK propagate the style from the triggering element to
the generated menu, we have to make sure .menubar .menuitem { }
selectors don't override the actual menuitem style.

cosimoc/gnome-themes-standard,rgcjonas/adwaita-semidark,cosimoc/gnome-themes-standard,cosimoc/gnome-themes-standard,magcius/gnome-themes-standard,rgcjonas/adwaita-semidark,magcius/gnome-themes-standard,magcius/gnome-themes-standard,ChrisCummins/gnome-themes-standard,rgcjonas/adwaita-semidark,ChrisCummins/gnome-themes-standard,ChrisCummins/gnome-themes-standard,cosimoc/gnome-themes-standard,Distrotech/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,ChrisCummins/gnome-themes-standard,DoctorJellyface/gnome-themes-standard,rgcjonas/adwaita-semidark,Distrotech/gnome-themes-standard,ChrisCummins/gnome-themes-standard