Sets a custom title for the header bar.
The title should help a user identify the current view. This supersedes any title set by [[email protected]_title] or [ [email protected]_subtitle]. To achieve the same style as the builtin title and subtitle, use the `.title` and `.subtitle` style classes.
You should set the custom title to `NULL`, for the header title label to be visible again.
| this |
a header bar |
| title_widget |
a custom widget to use for a title |