Adds a header bar to a HeaderGroup.
The decoration layout of the widgets will be edited depending on their position in the composite header bar, the start widget displaying only the start of the user's decoration layout and the end widget displaying only its end while widgets in the middle won't display anything. A header bar can be set as having the focus to display all the decorations. See set_decoration_layout .
When the widget is destroyed or no longer referenced elsewhere, it will be removed from the header group.
| this | |
| header_bar |
the HeaderBar to add |