If focus is not the current focus widget, and is focusable, sets it as the focus widget for the root.
If focus is null, unsets the focus widget for the root.
To set the focus to a particular widget in the root, it is usually more convenient to use [[email protected]_focus] instead of this function.
| this |
a `GtkRoot` |
| focus |
widget to be the new focus widget, or null to unset the focus widget |