Gets the `GtkCellRenderer` which is expected to be focusable for which renderer
is, or may be a sibling.
This is handy for `GtkCellArea` subclasses when handling events, after determining the renderer at the event location it can then chose to activate the focus cell for which the event cell may have been a sibling.
this |
a `GtkCellArea` |
renderer |
the `GtkCellRenderer` |
the `GtkCellRenderer` for which |