notebook: flip padding according to the tab position
notebook: flip padding according to the tab position

Now that GtkNotebook supports indicating the tab position with a style
class, use it to flip the padding if the tab is on the bottom side.

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