gtk2: use new primary-button-warps-slider style property
gtk2: use new primary-button-warps-slider style property

This makes the behavior of GTK2 scrollbars consistent with what we have
for GTK3.

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