the prefix string, typically used by the user, which is compared with each of the items
if non-NULL, returns the longest prefix which is common to all items that matched prefix, or NULL if no items matched prefix. This string should be freed when no longer needed.
the list of items whose strings begin with prefix. This should not be changed. [element-type utf8][transfer none]
Warning g_completion_complete_utf8 has been deprecated since version 2.26 and should not be used in newly-written code. Rarely used API Attempts to complete the string prefix using the GCompletion target items. In contrast to g_completion_complete(), this function returns the largest common prefix that is a valid UTF-8 string, omitting a possible common partial character. You should use this function instead of g_completion_complete() if your items are UTF-8 strings. Since 2.4