get_categories_list
Description:
Queries the list of categories of a calendar component object.
Each element in the returned categ_list is a string with the corresponding category. Free the returned SList with g_slist_free_full (categories, g_free); , when no longer needed.
Parameters:
| this | A calendar component object. | 
Returns:
| the SList of strings, where each string is a category, or null, when no category is set. |