the prefix string, typically typed 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. [transfer none]
Warning g_completion_complete 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.