Add more introspection capabilities to the base model class
* clutter/clutter-model.h:
* clutter/clutter-model.c:
(clutter_model_finalize): Do no leak the column types array.
(clutter_model_get_n_columns): Function for getting the number
of columns inside a model.
(clutter_model_set_names): Add API for setting the names of the
columns.
(clutter_model_get_column_name),
(_model_get_column_type): Add API for getting each column's name
and the default implementation for ClutterModel.
(clutter_model_get_column_type),
(_model_get_iter_at_row): Add API for getting each column's type
and the default implementation for ClutterModel.
* clutter.symbols: Add ClutterModel and ClutterModelIter symbols,
and keep the symbols file sorted.
