headerbar: support expand property for children

headerbar: support expand property for children

The header bar currently ignores the expand property on its
children. This commit changes the code to honor that property.

It divvies up any free space and distributes it equally to packed
children (with any left over space given out a pixel at a time
on a first come, first serve basis).

This commit also adds support for the title widget to be made
expandable.

It accomplishes this by using up the padding the title widget
is centered with.

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