void gtk_text_iter_set_line_offset ( GtkTextIter* iter, int char_on_line )
iter within a line, to a new character (not byte) offset.
The given character offset must be less than or equal to the number
of characters in the line; if equal,
iter moves to the start of the
next line. See
gtk_text_iter_set_line_index() if you have a byte
index rather than a character offset.