tab: tone down the inset shadow for :hover
Don't prelight active tabs

https://bugzilla.gnome.org/show_bug.cgi?id=722710
Remove bottom border from notebooks with show-border=FALSE

This leads to double borders in, for example, gtk3-demo. Show-border has an effect of adding .frame class to notebook, so the very next rule re-adds the border to bordered notebooks.

https://bugzilla.gnome.org/show_bug.cgi?id=722717