This mostly goes to keep consistency with the changes to GtkSizeRequest
in the last patch, as GtkCellSizeRequest requires GtkCellRenderer and
GtkCellRenderer implements GtkCellSizeRequest there's no use in keeping
them separate.

This patch renames the functions:
gtk_cell_size_request_get_request_mode()
  => gtk_cell_renderer_get_request_mode()
gtk_cell_size_request_get_width()
  => gtk_cell_renderer_get_preferred_width()
gtk_cell_size_request_get_height()
  => gtk_cell_renderer_get_preferred_height()
gtk_cell_size_request_get_size()
  => gtk_cell_renderer_get_preferred_size()
gtk_cell_size_request_get_width_for_height()
  => gtk_cell_renderer_get_preferred_width_for_height()
gtk_cell_size_request_get_height_for_width()
  => gtk_cell_renderer_get_preferred_height_for_width()
... and moves the corresponding vfuncs to GtkCellRenderer.

The patch also renames the implementations of these functions in cell
renderers to include the word "preferrred".