notebook: Include arrow size in tab size computation
notebook: Include arrow size in tab size computation

We don't want tabs to resize when a 2nd page is added, just because the
scroll arrows are now visible. And we do want the arrow size to be
included.

Note: Previosuly this code was never run, as the check
  requisition->width < tab_width
was only run when tab_width wasn't computed, so was still equal to 0
(same for the height). So what this patch essentially does it add the
functionality of the if switch.

grubersjoe/adwaita,jadahl/gtk,ahodesuka/gtk,jadahl/gtk,ahodesuka/gtk,Adamovskiy/gtk,alexlarsson/gtk,Adamovskiy/gtk,msteinert/gtk,Lyude/gtk-,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,ebassi/gtk,jigpu/gtk,jigpu/gtk,davidt/gtk,Distrotech/gtk2,ebassi/gtk,chergert/gtk,jigpu/gtk,Lyude/gtk-,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,Lyude/gtk-,alexlarsson/gtk,Lyude/gtk-,grubersjoe/adwaita,davidgumberg/gtk,Sidnioulz/SandboxGtk,davidt/gtk,Adamovskiy/gtk,davidgumberg/gtk,msteinert/gtk,bratsche/gtk-,alexlarsson/gtk,Distrotech/gtk2,jigpu/gtk,bratsche/gtk-,jigpu/gtk,ahodesuka/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,Sidnioulz/SandboxGtk,Distrotech/gtk2,Sidnioulz/SandboxGtk,ahodesuka/gtk,msteinert/gtk,jadahl/gtk,davidt/gtk,davidgumberg/gtk,davidgumberg/gtk,Lyude/gtk-,jessevdk/gtk,Adamovskiy/gtk,grubersjoe/adwaita,bratsche/gtk-,davidgumberg/gtk,jessevdk/gtk,davidt/gtk,grubersjoe/adwaita,alexlarsson/gtk,jigpu/gtk,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,jessevdk/gtk,chergert/gtk,Distrotech/gtk2,msteinert/gtk,Distrotech/gtk2,Adamovskiy/gtk,jigpu/gtk,ebassi/gtk,bratsche/gtk-,Adamovskiy/gtk,bratsche/gtk-,jigpu/gtk,chergert/gtk,ahodesuka/gtk,jessevdk/gtk,ebassi/gtk,Distrotech/gtk2,chergert/gtk,Adamovskiy/gtk,davidt/gtk,jadahl/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,jadahl/gtk,msteinert/gtk,davidgumberg/gtk,chergert/gtk,bratsche/gtk-,Lyude/gtk-,davidt/gtk,chergert/gtk,jadahl/gtk,alexlarsson/gtk,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,Lyude/gtk-,Sidnioulz/SandboxGtk,jadahl/gtk,davidgumberg/gtk,ahodesuka/gtk,ebassi/gtk,jessevdk/gtk,jessevdk/gtk,chergert/gtk,jessevdk/gtk,alexlarsson/gtk,ebassi/gtk