Fix notebook tab close buttons with new Gtk+
Fix notebook tab close buttons with new Gtk+

Gtk+ no longer propagates :hover state to child widgets, so we
can't do something like ".button GtkImage:hover". Fortunately the
stuff the code was using is all inherited properties that
we may as well set on the button itself instead.

https://bugzilla.gnome.org/show_bug.cgi?id=708144

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