notebook: inform the theme whether show-borders was set
notebook: inform the theme whether show-borders was set

So that it can draw a border or not. If show-borders=true then
the notebook will have GTK_STYLE_CLASS_FRAME.

jadahl/gtk,jessevdk/gtk,davidgumberg/gtk,chergert/gtk,jigpu/gtk,alexlarsson/gtk,jigpu/gtk,jigpu/gtk,davidgumberg/gtk,jessevdk/gtk,grubersjoe/adwaita,alexlarsson/gtk,Adamovskiy/gtk,davidgumberg/gtk,alexlarsson/gtk,jessevdk/gtk,ahodesuka/gtk,jigpu/gtk,davidgumberg/gtk,Adamovskiy/gtk,chergert/gtk,Sidnioulz/SandboxGtk,ahodesuka/gtk,msteinert/gtk,jigpu/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jigpu/gtk,ahodesuka/gtk,jessevdk/gtk,jessevdk/gtk,jessevdk/gtk,grubersjoe/adwaita,jigpu/gtk,msteinert/gtk,jadahl/gtk,jadahl/gtk,Lyude/gtk-,Adamovskiy/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,jadahl/gtk,Adamovskiy/gtk,ahodesuka/gtk,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,ahodesuka/gtk,Lyude/gtk-,jigpu/gtk,davidgumberg/gtk,Lyude/gtk-,grubersjoe/adwaita,alexlarsson/gtk,Lyude/gtk-,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,davidgumberg/gtk,grubersjoe/adwaita,Adamovskiy/gtk,alexlarsson/gtk,davidgumberg/gtk,alexlarsson/gtk,chergert/gtk,chergert/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,chergert/gtk,jadahl/gtk,ahodesuka/gtk,msteinert/gtk,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,Lyude/gtk-,Lyude/gtk-,msteinert/gtk,Sidnioulz/SandboxGtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,Adamovskiy/gtk,jessevdk/gtk,davidgumberg/gtk,chergert/gtk