Add a new GtkSourceCompletionProvider to the completion object. This will add a reference provider, so make sure to unref your own copy when you no longer need it.
A GtkSourceCompletionProvider
TRUE if provider was successfully added, otherwise if error is provided, it will be set with the error and FALSE is returned.
GException on failure.
See Implementation
Add a new GtkSourceCompletionProvider to the completion object. This will add a reference provider, so make sure to unref your own copy when you no longer need it.