gtk-cell-area-add-focus-sibling (area renderer sibling)

area -- a gtk-cell-area object
renderer -- a gtk-cell-renderer object expected to have focus
sibling -- a gtk-cell-renderer object to add to the focus area of the cell renderer

Adds sibling to focusable area of the cell renderer, 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.