headerbar: compute nominal size of sides up front

headerbar: compute nominal size of sides up front

In order to support the expand property on children, we're
going to need to look at the size of the packed children
on each side of the title widget, up front, before allocating
them (to compute how much extra allocation each expanded child
gets).

This commit lays the groundwork for that analysis by splitting
the size calculation of each side of the header bar outside
of the loop that allocates each child of the header bar.

https://bugzilla.gnome.org/show_bug.cgi?id=724332