Add 1px height to HeaderBar image buttons
Add 1px height to HeaderBar image buttons

This makes them the same height as text buttons, and accomodates
the double stroke on the bottom edge.

https://bugzilla.gnome.org/show_bug.cgi?id=706644

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