a datalist.
the GQuark to identify the data element.
the data element or NULL to remove any previous element corresponding to key_id.
the function to call when the data element is removed. This function will be called with the data element and can be used to free any memory allocated for it. If data is NULL, then destroy_func must also be NULL.
Sets the data corresponding to the given GQuark id, and the function to be called when the element is removed from the datalist. Any previous data with the same key is removed, and its destroy function is called.