Str.strcanon

For each character in string, if the character is not in valid_chars, replaces the character with substitutor. Modifies string in place, and return string itself, not a copy. The return value is to allow nesting such as

g_ascii_strup (g_strcanon (str, "abc", '?'))

In order to modify a copy, you may use `g_strdup()`:

reformatted = g_strcanon (g_strdup (const_str), "abc", '?');
...
g_free (reformatted);