Focus driving in GtkCellArea now works.

Focus driving in GtkCellArea now works.

 - Fixed focus driving in GtkCellArea with refined apis
 - Added gtk_cell_area_activate() to be called when the area has focus
   (to activate or start editing the focused cell)
 - Added support for this in cellareascaffold
 - testcellarea now watches the "toggled" signal for a toggle renderer
   and updates the model state accordingly, this currently works with
   keyboard navigation, however focus is still not painted on cells.