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
csd: don't override border and box shadow for titlebars

Since .titlebar is added by default to all the widgets used as titles
for a toplevel, we can't override the border image or the box shadow as,
for instance, headerbars want to render those.
A following commit will remove the borders explicitly in case the client
side decoration is the default one added automatically by GTK.

https://bugzilla.gnome.org/show_bug.cgi?id=706418
csd: don't draw a border for automatic CSD titles added by GTK

For those we want to be consistent with the mutter theme we have
nowadays, so use the extra style class to distinguish that case, and
remove any border there.

https://bugzilla.gnome.org/show_bug.cgi?id=706418