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.