get_focus_from_sibling

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.

Parameters:

this	a `GtkCellArea`
renderer	the `GtkCellRenderer`

Returns:

the `GtkCellRenderer` for which renderer is a sibling