Create a new #GtkSourceCompletionContext for @completion. The position where
the completion occurs can be specified by @position. If @position is %NULL,
the current cursor position will be used.
a new #GtkSourceCompletionContext.
The reference being returned is a 'floating' reference,
so if you invoke gtk_source_completion_start() with this context
you don't need to unref it.
Create a new #GtkSourceCompletionContext for @completion. The position where the completion occurs can be specified by @position. If @position is %NULL, the current cursor position will be used.