API CHANGE: editable and activatable cell renderers

[Jonathan Blandford <jrb redhat com>]

It turns out that activatable and editable cell renderers are fairly
useless without the model and path passed into them.  We are passing in
the path in the form of a string, but not the model:

For example:

  void (* toggled) (GtkCellRendererToggle *cell_renderer_toggle,
		    gchar                 *path);
  void (* edited)  (GtkCellRendererText *cell_renderer_text,
		    gchar               *path,
		    gchar               *new_text);

I would like to add a (gpointer model) argument to these two renderers
so that you don't need to pass the model in as user_data or by using
g_object_set_user_data.

Does anyone think that model is a bad argument name?  Currently
CellRenderers know nothing of the GtkTreeView/Model, and are fairly
generic.  For example, a Sheet widget could use them.  However, not all
possible uses of the renderer would necessarily have a model.

Thanks,
-Jonathan