header bar: Make it possible to have no title

header bar: Make it possible to have no title

Add a custom title had the side-effect of showing the widget.
That is not right, adding children and managing their visibility
should be independent. The headerbar size allocation code also
made the assumption that a custom title is always visible.

With these changes, GtkHeaderBar should be usable in situations
where the centering functionality is not required, and it is
important to freely pack content at both ends, such as in nautilus.

https://bugzilla.gnome.org/show_bug.cgi?id=722340