Undocumented in source.
GtkHeaderBar is similar to a horizontal gtk.Box It allows children to be placed at the start or the end. In addition, it allows a title and subtitle to be displayed. The title will be centered with respect to the width of the box, even if the children at either side take up different amounts of space. The height of the titlebar will be set to provide sufficient space for the subtitle, even if none is currently set. If a subtitle is not needed, the space reservation can be turned off with HeaderBar.setHasSubtitle.