headerbar: simplify some size allocation code

headerbar: simplify some size allocation code

With a headerbar, the widget in the center may be a label, constructed
internally, or a custom widget, constructed externally.  The size
allocation code needs to handle either case the same way for the most
part.  There's more than one place in the code that checks which of
the two widgets to use and does some operation on the selected one.

This commit simplifies the code by checking up front which one is the
center (title) widget and storing that in a temporary variable,
This allows reducing duplicated logic later on in the function.

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