Fix up padding inside header bars

Image buttons had 1px less padding on the right. This meant
that they weren't square. It might have been caused by confusion
about the uneven bottom padding in header bars, so add a comment
about that.

Also, remove the extra 1px padding inside header bars, to account
for the extra line coming from the border.

https://bugzilla.gnome.org/show_bug.cgi?id=708802