Added gtk_cell_area_foreach_alloc() to further simplify GtkCellArea subclasses

Added gtk_cell_area_foreach_alloc() to further simplify GtkCellArea subclasses

gtk_cell_area_foreach_alloc() itterates over an allocated list of cells
and calls a callback with the cell's allocation until the callback returns
TRUE.
  - Removed vfunc get_cell_allocation() and implemented
    gtk_cell_area_get_cell_allocation() using foreach_alloc()
  - Added gtk_cell_area_get_cell_at_position() using foreach_alloc()
  - Removed GtkCellAreaBox ->event() implementation and implemented
    mouse "click" cell activation in GtkCellArea class directly using
    gtk_cell_area_get_cell_at_position().