Ironed out the kinks in editing apis for GtkCellArea

Ironed out the kinks in editing apis for GtkCellArea

 - Added gtk_cell_area_aligned_cell_area() to get the aligned
   internal area use by a cell (for focus painting and for
   event areas).

 - Provide the event area in "editing-started" signal

 - Fire "remove-editable" when editing is canceled by the user,
   an implementing layouting widget need only catch "editing-started"
   and "remove-editable" now.

 - CellAreaScaffold/testcellarea now edit textrenderers.