Adds sibling to renderer’s focusable area, focus will be drawn around renderer and all of its
siblings if renderer can focus for a given row.
Events handled by focus siblings can also activate the given focusable renderer.
| this |
a `GtkCellArea` |
| renderer |
the `GtkCellRenderer` expected to have focus |
| sibling |
the `GtkCellRenderer` to add to |