notebook: adapt to engine changes
notebook: adapt to engine changes

We can simplify the CSS a bit and use linear-gradient to draw the active
tab highlight with pixel-precision.

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