adwaita: improve GTK2 fallback menu stroke gradient
adwaita: improve GTK2 fallback menu stroke gradient

Add some color stops to make the outline gradient more refined.

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